rgbds/src/asm/rgbasm.5

.\"
.\" This file is part of RGBDS.
.\"
.\" Copyright (c) 2017-2018, Antonio Nino Diaz and RGBDS contributors.
.\"
.\" SPDX-License-Identifier: MIT
.\"
.Dd December 5, 2019
.Dt RGBASM 5
.Os
.Sh NAME
.Nm rgbasm
.Nd language documentation
.Sh DESCRIPTION
This is the full description of the language used by
.Xr rgbasm 1 .
The description of the instructions supported by the Game Boy CPU is in
.Xr gbz80 7 .
.Pp
It is strongly recommended to have some familiarity with the Game Boy hardware before reading this document.
RGBDS is specifically targeted at the Game Boy, and thus a lot of its features tie directly to its concepts.
This document is not intended to be a Game Boy hardware reference.
.Pp
Generally,
.Dq the linker
will refer to
.Xr rgblink 1 ,
but any program that processes RGB object files (described in
.Xr rgbds 5 )
can be used in its place.
.Sh SYNTAX
The syntax is line‐based, just as in any other assembler, meaning that you do one instruction or directive per line:
.Pp
.Dl Oo Ar label Oc Oo Ar instruction Oc Oo Ar ;\ comment Oc
.Pp
Example:
.Bd -literal -offset indent
John: ld a,87 ;Weee
.Ed
.Pp
All reserved keywords (directives, mnemonics, registers, etc.) are case‐insensitive;
all identifiers (symbol names) are case-sensitive.
.Pp
Comments are used to give humans information about the code, such as explanations.
The assembler
.Em always
ignores comments and their contents.
.Pp
There are two syntaxes for comments.
The most common is that anything that follows a semicolon
.Ql \&;
not inside a string, is a comment until the end of the line.
The second is a block comment, beginning with
.Ql /*
and ending with
.Ql */ .
It can be split across multiple lines, or occur in the middle of an expression:
.Bd -literal -offset indent
X = /* the value of x
       should be 3 */ 3
.Ed
.Pp
Sometimes lines can be too long and it may be necessary to split them.
To do so, put a backslash at the end of the line:
.Bd -literal -offset indent
    DB 1, 2, 3,\ \[rs]
       4, 5, 6,\ \[rs]\ ;\ Put it before any comments
       7, 8, 9
.Ed
.Pp
This works anywhere in the code except inside of strings.
To split strings it is needed to use
.Fn STRCAT
like this:
.Bd -literal -offset indent
    db STRCAT("Hello ",\ \[rs]
              "world!")
.Ed
.Sh EXPRESSIONS
An expression can be composed of many things.
Numerical expressions are always evaluated using signed 32-bit math.
Zero is considered to be the only "false" number, all non-zero numbers (including negative) are "true".
.Pp
An expression is said to be "constant" if
.Nm
knows its value.
This is generally always the case, unless a label is involved, as explained in the
.Sx SYMBOLS
section.
.Pp
The instructions in the macro-language generally require constant expressions.
.Ss Numeric Formats
There are a number of numeric formats.
.Bl -column -offset indent "Fixed point (16.16)" "Prefix"
.It Sy Format type Ta Sy Prefix Ta Sy Accepted characters
.It Hexadecimal Ta $ Ta 0123456789ABCDEF
.It Decimal Ta none Ta 0123456789
.It Octal Ta & Ta 01234567
.It Binary Ta % Ta 01
.It Fixed point (16.16) Ta none Ta 01234.56789
.It Character constant Ta none Ta \(dqABYZ\(dq
.It Gameboy graphics Ta \` Ta 0123
.El
.Pp
Underscores are also accepted in numbers, except at the beginning of one.
.Pp
The "character constant" form yields the value the character maps to in the current charmap.
For example, by default
.Pq refer to Xr ascii 7
.Sq \(dqA\(dq
yields 65.
See
.Sx Character maps
for information on charmaps.
.Pp
The last one, Gameboy graphics, is quite interesting and useful.
After the backtick, 8 digits between 0 and 3 are expected, corresponding to pixel values.
The resulting value is the two bytes of tile data that would produce that row of pixels.
For example,
.Sq \`01012323
is equivalent to
.Sq $0F55 .
.Pp
You can also use symbols, which are implicitly replaced with their value.
.Ss Operators
A great number of operators you can use in expressions are available (listed from highest to lowest precedence):
.Bl -column -offset indent "!= == <= >= < >"
.It Sy Operator Ta Sy Meaning
.It Li \&( \&) Ta Precedence override
.It Li FUNC() Ta Built-in function call
.It Li ** Ta Exponent
.It Li ~ + - Ta Unary complement/plus/minus
.It Li * / % Ta Multiply/divide/modulo
.It Li << >> Ta Shift left/right
.It Li & \&| ^ Ta Binary and/or/xor
.It Li + - Ta Add/subtract
.It Li != == <= >= < > Ta Comparison
.It Li && || Ta Boolean and/or
.It Li \&! Ta Unary not
.El
.Pp
.Ic ~
complements a value by inverting all its bits.
.Pp
.Ic %
is used to get the remainder of the corresponding division, so that
.Sq a / b * b + a % b == a
is always true.
The result has the same sign as the divisor.
This makes
.Sq a % b .
equal to
.Sq (a + b) % b
or
.Sq (a - b) % b .
.Pp
Shifting works by shifting all bits in the left operand either left
.Pq Sq <<
or right
.Pq Sq >>
by the right operand's amount.
When shifting left, all newly-inserted bits are reset; when shifting right, they are copies of the original most significant bit instead.
This makes
.Sq a << b
and
.Sq a >> b
equivalent to multiplying and dividing by 2 to the power of b, respectively.
.Pp
Comparison operators return 0 if the comparison is false, and 1 otherwise.
.Pp
Unlike in a lot of languages, and for technical reasons,
.Nm
still evaluates both operands of
.Sq &&
and
.Sq || .
.Pp
.Ic \&!
returns 1 if the operand was 0, and 0 otherwise.
.Ss Fixed‐point Expressions
Fixed-point numbers are basically normal (32-bit) integers, which count 65536th's instead of entire units, offering better precision than integers but limiting the range of values.
The upper 16 bits are used for the integer part and the lower 16 bits are used for the fraction (65536ths).
Since they are still akin to integers, you can use them in normal integer expressions, and some integer operators like
.Sq +
and
.Sq -
don't care whether the operands are integers or fixed-point.
You can easily truncate a fixed-point number into an integer by shifting it right by 16 bits.
It follows that you can convert an integer to a fixed-point number by shifting it left.
.Pp
The following functions are designed to operate with fixed-point numbers:
.EQ
delim $$
.EN
.Bl -column -offset indent "ATAN2(x, y)"
.It Sy Name Ta Sy Operation
.It Fn DIV x y Ta $x \[di] y$
.It Fn MUL x y Ta $x \[mu] y$
.It Fn POW x y Ta $x$ to the $y$ power
.It Fn LOG x y Ta Logarithm of $x$ to the base $y$
.It Fn ROUND x Ta Round $x$ to the nearest integer
.It Fn CEIL x Ta Round $x$ up to an integer
.It Fn FLOOR x Ta Round $x$ down to an integer
.It Fn SIN x Ta Sine of $x$
.It Fn COS x Ta Cosine of $x$
.It Fn TAN x Ta Tangent of $x$
.It Fn ASIN x Ta Inverse sine of $x$
.It Fn ACOS x Ta Inverse cosine of $x$
.It Fn ATAN x Ta Inverse tangent of $x$
.It Fn ATAN2 x y Ta Angle between $( x , y )$ and $( 1 , 0 )$
.El
.EQ
delim off
.EN
.Pp
The trigonometry functions (
.Ic SIN ,
.Ic COS ,
.Ic TAN ,
etc) are defined in terms of a circle divided into 65535.0 degrees.
.Pp
These functions are useful for automatic generation of various tables.
For example:
.Bd -literal -offset indent
; Generate a 256-byte sine table with values in the range [0, 128]
; (shifted and scaled from the range [-1.0, 1.0])
ANGLE = 0.0
    REPT 256
        db (MUL(64.0, SIN(ANGLE)) + 64.0) >> 16
ANGLE = ANGLE + 256.0 ; 256.0 = 65536 degrees / 256 entries
    ENDR
.Ed
.Ss String Expressions
The most basic string expression is any number of characters contained in double quotes
.Pq Ql \&"for instance" .
The backslash character
.Ql \[rs]
is special in that it causes the character following it to be
.Dq escaped ,
meaning that it is treated differently from normal.
There are a number of escape sequences you can use within a string:
.Bl -column -offset indent "Qo \[rs]1 Qc \[en] Qo \[rs]9 Qc"
.It Sy String Ta Sy Meaning
.It Ql \[rs]\[rs] Ta Produces a backslash
.It Ql \[rs]" Ta Produces a double quote without terminating
.It Ql \[rs]{ Ta Curly bracket left
.It Ql \[rs]} Ta Curly bracket right
.It Ql \[rs]n Ta Newline ($0A)
.It Ql \[rs]r Ta Carriage return ($0D)
.It Ql \[rs]t Ta Tab ($09)
.It Qo \[rs]1 Qc \[en] Qo \[rs]9 Qc Ta Macro argument (Only in the body of a macro; see Sx Invoking macros )
.It Ql \[rs]# Ta All Dv _NARG No macro arguments, separated by commas (Only in the body of a macro)
.It Ql \[rs]@ Ta Label name suffix (Only in the body of a macro or a Ic REPT No block)
.El
(Note that some of those can be used outside of strings, when noted further in this document.)
.Pp
Multi-line strings are contained in triple quotes
.Pq Ql \&"\&"\&"for instance\&"\&"\&" .
Escape sequences work the same way in multi-line strings; however, literal newline
characters will be included as-is, without needing to escape them with
.Ql \[rs]r
or
.Ql \[rs]n .
.Pp
A funky feature is
.Ql {symbol}
within a string, called
.Dq symbol interpolation .
This will paste
.Ar symbol Ap s
contents as a string.
If it's a string symbol, the string is simply inserted.
If it's a numeric symbol, its value is converted to hexadecimal notation with a dollar sign
.Sq $
prepended.
.Bd -literal -offset indent
TOPIC equs "life, the universe, and \[rs]"everything\[rs]""
ANSWER = 42
;\ Prints "The answer to life, the universe, and "everything" is $2A"
PRINTLN "The answer to {TOPIC} is {ANSWER}"
.Ed
.Pp
Symbol interpolations can be nested, too!
.Pp
It's possible to change the way symbols are converted by specifying a print format like so:
.Ql {fmt:symbol} .
The
.Ql fmt
specifier consists of parts
.Ql <sign><prefix><align><pad><width><frac><type> .
These parts are:
.Bl -column "<prefix>"
.It Sy Part Ta Sy Meaning
.It Ql <sign> Ta May be
.Ql +
or
.Ql \  .
If specified, prints this character in front of non-negative numbers.
.It Ql <prefix> Ta May be
.Ql # .
If specified, prints the appropriate prefix for numbers,
.Ql $ ,
.Ql & ,
or
.Ql % .
.It Ql <align> Ta May be
.Ql - .
If specified, aligns left instead of right.
.It Ql <pad> Ta May be
.Ql 0 .
If specified, pads right-aligned numbers with zeros instead of spaces.
.It Ql <width> Ta May be one or more
.Ql 0
\[en]
.Ql 9 .
If specified, pads the value to this width, right-aligned with spaces by default.
.It Ql <frac> Ta May be
.Ql \&.
followed by one or more
.Ql 0
\[en]
.Ql 9 .
If specified, prints this many digits of a fixed-point fraction.
Defaults to 5 digits.
.It Ql <type> Ta Specifies the type of value.
.El
.Pp
All the format specifier parts are optional except the
.Ql <type> .
Valid print types are:
.Bl -column -offset indent "Print type" "Lowercase hexadecimal" "Example"
.It Sy Print type Ta Sy Format Ta Sy Example
.It Ql d Ta Signed decimal Ta -42
.It Ql u Ta Unsigned decimal Ta 42
.It Ql x Ta Lowercase hexadecimal Ta 2a
.It Ql X Ta Uppercase hexadecimal Ta 2A
.It Ql b Ta Binary Ta 101010
.It Ql o Ta Octal Ta 52
.It Ql f Ta Fixed-point Ta 1234.56789
.It Ql s Ta String Ta \&"example\&"
.El
.Pp
Examples:
.Bd -literal -offset indent
; Prints "%0010 + $3 == 5"
PRINTLN STRFMT("%#05b + %#x == %d", 2, 3, 2+3)
; Prints "32% of 20 = 6.40"
PRINTLN STRFMT("%d%% of %d = %.2f", 32, 20, MUL(20.0, 0.32))
; Prints "Hello world!"
PRINTLN STRFMT("Hello %s!", STRLWR("WORLD"))
.Ed
.Pp
HINT: The
.Ic {symbol}
construct can also be used outside strings.
The symbol's value is again inserted directly.
.Bd -literal -offset indent
NAME equs "ITEM"
FMT equs "d"
ZERO_NUM equ 0
ZERO_STR equs "0"
;\ Defines INDEX as 100
INDEX = 1{ZERO_STR}{{FMT}:ZERO_NUM}
;\ Defines ITEM_100 as "\[rs]"hundredth\[rs]""
{NAME}_{d:INDEX} equs "\[rs]"hundredth\[rs]""
;\ Prints "ITEM_100 is hundredth"
PRINTLN STRCAT("{NAME}_{d:INDEX} is ", {NAME}_{d:INDEX})
;\ Purges ITEM_100
PURGE {NAME}_{d:INDEX}
.Ed
.Pp
The following functions operate on string expressions.
Most of them return a string, however some of these functions actually return an integer and can be used as part of an integer expression!
.Bl -column "STRSUB(str, pos, len)"
.It Sy Name Ta Sy Operation
.It Fn STRLEN str Ta Returns the number of characters in Ar str .
.It Fn STRCAT strs... Ta Concatenates Ar strs .
.It Fn STRCMP str1 str2 Ta Returns -1 if Ar str1 No is alphabetically lower than Ar str2 No , zero if they match, 1 if Ar str1 No is greater than Ar str2 .
.It Fn STRIN str1 str2 Ta Returns the first position of Ar str2 No in Ar str1 No or zero if it's not present Pq first character is position 1 .
.It Fn STRRIN str1 str2 Ta Returns the last position of Ar str2 No in Ar str1 No or zero if it's not present Pq first character is position 1 .
.It Fn STRSUB str pos len Ta Returns a substring from Ar str No starting at Ar pos Po first character is position 1 Pc and Ar len No characters long.
.It Fn STRUPR str Ta Returns Ar str No with all letters in uppercase.
.It Fn STRLWR str Ta Returns Ar str No with all letters in lowercase.
.It Fn STRRPL str old new Ta Returns Ar str No with each non-overlapping occurrence of the substring Ar old No replaced with Ar new .
.It Fn STRFMT fmt args... Ta Returns the string Ar fmt No with each
.Ql %spec
pattern replaced by interpolating the format
.Ar spec
with its corresponding argument in
.Ar args
.Pq So %% Sc is replaced by the So % Sc character .
.El
.Ss Character maps
When writing text that is meant to be displayed in the Game Boy, the characters used in the source code may have a different encoding than the default of ASCII.
For example, the tiles used for uppercase letters may be placed starting at tile index 128, which makes it difficult to add text strings to the ROM.
.Pp
Character maps allow mapping strings up to 16 characters long to an abitrary 8-bit value:
.Bd -literal -offset indent
CHARMAP "<LF>", 10
CHARMAP "&iacute", 20
CHARMAP "A", 128
.Ed
By default, a character map contains ASCII encoding.
.Pp
It is possible to create multiple character maps and then switch between them as desired.
This can be used to encode debug information in ASCII and use a different encoding for other purposes, for example.
Initially, there is one character map called
.Sq main
and it is automatically selected as the current character map from the beginning.
There is also a character map stack that can be used to save and restore which character map is currently active.
.Bl -column "NEWCHARMAP name, basename"
.It Sy Command Ta Sy Meaning
.It Ic NEWCHARMAP Ar name Ta Creates a new, empty character map called Ar name .
.It Ic NEWCHARMAP Ar name , basename Ta Creates a new character map called Ar name , No copied from character map Ar basename .
.It Ic SETCHARMAP Ar name Ta Switch to character map Ar name .
.It Ic PUSHC Ta Push the current character map onto the stack.
.It Ic POPC Ta Pop a character map off the stack and switch to it.
.El
.Pp
.Sy Note:
Character maps affect all strings in the file from the point in which they are defined, until switching to a different character map.
This means that any string that the code may want to print as debug information will also be affected by it.
.Pp
.Sy Note:
The output value of a mapping can be 0.
If this happens, the assembler will treat this as the end of the string and the rest of it will be trimmed.
.Ss Other functions
There are a few other functions that do various useful things:
.Bl -column "DEF(symbol)"
.It Sy Name Ta Sy Operation
.It Fn BANK arg Ta Returns a bank number.
If
.Ar arg
is the symbol
.Ic @ ,
this function returns the bank of the current section.
If
.Ar arg
is a string, it returns the bank of the section that has that name.
If
.Ar arg
is a label, it returns the bank number the label is in.
The result may be constant if
.Nm
is able to compute it.
.It Fn DEF symbol Ta Returns TRUE (1) if
.Ar symbol
has been defined, FALSE (0) otherwise.
String symbols are not expanded within the parentheses.
.It Fn HIGH arg Ta Returns the top 8 bits of the operand if Ar arg No is a label or constant, or the top 8-bit register if it is a 16-bit register.
.It Fn LOW arg Ta Returns the bottom 8 bits of the operand if Ar arg No is a label or constant, or the bottom 8-bit register if it is a 16-bit register Pq Cm AF No isn't a valid register for this function .
.It Fn ISCONST arg Ta Returns 1 if Ar arg Ap s value is known by RGBASM (e.g. if it can be an argument to
.Ic IF ) ,
or 0 if only RGBLINK can compute its value.
.El
.Sh SECTIONS
Before you can start writing code, you must define a section.
This tells the assembler what kind of information follows and, if it is code, where to put it.
.Pp
.Dl SECTION Ar name , type
.Dl SECTION Ar name , type , options
.Dl SECTION Ar name , type Ns Bo Ar addr Bc
.Dl SECTION Ar name , type Ns Bo Ar addr Bc , Ar options
.Pp
.Ar name
is a string enclosed in double quotes, and can be a new name or the name of an existing section.
If the type doesn't match, an error occurs.
All other sections must have a unique name, even in different source files, or the linker will treat it as an error.
.Pp
Possible section
.Ar type Ns s
are as follows:
.Bl -tag
.It Ic ROM0
A ROM section.
.Ar addr
can range from
.Ad $0000
to
.Ad $3FFF ,
or
.Ad $0000
to
.Ad $7FFF
if tiny ROM mode is enabled in the linker.
.It Ic ROMX
A banked ROM section.
.Ar addr
can range from
.Ad $4000
to
.Ad $7FFF .
.Ar bank
can range from 1 to 511.
Becomes an alias for
.Ic ROM0
if tiny ROM mode is enabled in the linker.
.It Ic VRAM
A banked video RAM section.
.Ar addr
can range from
.Ad $8000
to
.Ad $9FFF .
.Ar bank
can be 0 or 1, but bank 1 is unavailable if DMG mode is enabled in the linker.
.It Ic SRAM
A banked external (save) RAM section.
.Ar addr
can range from
.Ad $A000
to
.Ad $BFFF .
.Ar bank
can range from 0 to 15.
.It Ic WRAM0
A general-purpose RAM section.
.Ar addr
can range from
.Ad $C000
to
.Ad $CFFF ,
or
.Ad $C000
to
.Ad $DFFF
if WRAM0 mode is enabled in the linker.
.It Ic WRAMX
A banked general-purpose RAM section.
.Ar addr
can range from
.Ad $D000
to
.Ad $DFFF .
.Ar bank
can range from 1 to 7.
Becomes an alias for
.Ic WRAM0
if WRAM0 mode is enabled in the linker.
.It Ic OAM
An object attribute RAM section.
.Ar addr
can range from
.Ad $FE00
to
.Ad $FE9F .
.It Ic HRAM
A high RAM section.
.Ar addr
can range from
.Ad $FF80
to
.Ad $FFFE .
.Pp
.Sy Note :
While
.Nm
will automatically optimize
.Ic ld
instructions to the smaller and faster
.Ic ldh
(see
.Xr gbz80 7 )
whenever possible, it is generally unable to do so when a label is involved.
Using the
.Ic ldh
instruction directly is recommended.
This forces the assembler to emit a
.Ic ldh
instruction and the linker to check if the value is in the correct range.
.El
.Pp
Since RGBDS produces ROMs, code and data can only be placed in
.Ic ROM0
and
.Ic ROMX
sections.
To put some in RAM, have it stored in ROM, and copy it to RAM.
.Pp
.Ar option Ns s are comma-separated and may include:
.Bl -tag
.It Ic BANK Ns Bq Ar bank
Specify which
.Ar bank
for the linker to place the section in.
See above for possible values for
.Ar bank ,
depending on
.Ar type .
.It Ic ALIGN Ns Bq Ar align , offset
Place the section at an address whose
.Ar align
least‐significant bits are equal to
.Ar offset .
(Note that
.Ic ALIGN Ns Bq Ar align
is a shorthand for
.Ic ALIGN Ns Bq Ar align , No 0 ) .
This option can be used with
.Bq Ar addr ,
as long as they don't contradict eachother.
It's also possible to request alignment in the middle of a section, see
.Sx Requesting alignment
below.
.El
.Pp
If
.Bq Ar addr
is not specified, the section is considered
.Dq floating ;
the linker will automatically calculate an appropriate address for the section.
Similarly, if
.Ic BANK Ns Bq Ar bank
is not specified, the linker will automatically find a bank with enough space.
.Pp
Sections can also be placed by using a linker script file.
The format is described in
.Xr rgblink 5 .
They allow the user to place floating sections in the desired bank in the order specified in the script.
This is useful if the sections can't be placed at an address manually because the size may change, but they have to be together.
.Pp
Section examples:
.Bl -item
.It
.Bd -literal -offset indent
SECTION "Cool Stuff",ROMX
.Ed
This switches to the section called
.Dq CoolStuff ,
creating it if it doesn't already exist.
It can end up in any ROM bank.
Code and data may follow.
.It
If it is needed, the the base address of the section can be specified:
.Bd -literal -offset indent
SECTION "Cool Stuff",ROMX[$4567]
.Ed
.It
An example with a fixed bank:
.Bd -literal -offset indent
SECTION "Cool Stuff",ROMX[$4567],BANK[3]
.Ed
.It
And if you want to force only the section's bank, and not its position within the bank, that's also possible:
.Bd -literal -offset indent
SECTION "Cool Stuff",ROMX,BANK[7]
.Ed
.It
Alignment examples:
The first one could be useful for defining an OAM buffer to be DMA'd, since it must be aligned to 256 bytes.
The second could also be appropriate for GBC HDMA, or for an optimized copy code that requires alignment.
.Bd -literal -offset indent
SECTION "OAM Data",WRAM0,ALIGN[8] ;\ align to 256 bytes
SECTION "VRAM Data",ROMX,BANK[2],ALIGN[4] ;\ align to 16 bytes
.Ed
.El
.Ss Section Stack
.Ic POPS
and
.Ic PUSHS
provide the interface to the section stack.
The number of entries in the stack is limited only by the amount of memory in your machine.
.Pp
.Ic PUSHS
will push the current section context on the section stack.
.Ic POPS
can then later be used to restore it.
Useful for defining sections in included files when you don't want to override the section context at the point the file was included.
.Ss RAM Code
Sometimes you want to have some code in RAM.
But then you can't simply put it in a RAM section, you have to store it in ROM and copy it to RAM at some point.
.Pp
This means the code (or data) will not be stored in the place it gets executed.
Luckily,
.Ic LOAD
blocks are the perfect solution to that.
Here's an example of how to use them:
.Bd -literal -offset indent
SECTION "LOAD example", ROMX
CopyCode:
    ld de, RAMCode
    ld hl, RAMLocation
    ld c, RAMLocation.end - RAMLocation
\&.loop
    ld a, [de]
    inc de
    ld [hli], a
    dec c
    jr nz, .loop
    ret

RAMCode:
  LOAD "RAM code", WRAM0
RAMLocation:
    ld hl, .string
    ld de, $9864
\&.copy
    ld a, [hli]
    ld [de], a
    inc de
    and a
    jr nz, .copy
    ret

\&.string
    db "Hello World!", 0
\&.end
  ENDL
.Ed
.Pp
A
.Ic LOAD
block feels similar to a
.Ic SECTION
declaration because it creates a new one.
All data and code generated within such a block is placed in the current section like usual, but all labels are created as if they were placed in this newly-created section.
.Pp
In the example above, all of the code and data will end up in the "LOAD example" section.
You will notice the
.Sq RAMCode
and
.Sq RAMLocation
labels.
The former is situated in ROM, where the code is stored, the latter in RAM, where the code will be loaded.
.Pp
You cannot nest
.Ic LOAD
blocks, nor can you change the current section within them.
.Pp
.Ic LOAD
blocks can use the
.Ic UNION
or
.Ic FRAGMENT
modifiers, as described below.
.Ss Unionized Sections
When you're tight on RAM, you may want to define overlapping blocks of variables, as explained in the
.Sx Unions
section.
However, the
.Ic UNION
keyword only works within a single file, which prevents e.g. defining temporary variables on a single memory area across several files.
Unionized sections solve this problem.
To declare an unionized section, add a
.Ic UNION
keyword after the
.Ic SECTION
one; the declaration is otherwise not different.
Unionized sections follow some different rules from normal sections:
.Bl -bullet -offset indent
.It
The same unionized section (= having the same name) can be declared several times per
.Nm
invocation, and across several invocations.
Different declarations are treated and merged identically whether within the same invocation, or different ones.
.It
If one section has been declared as unionized, all sections with the same name must be declared unionized as well.
.It
All declarations must have the same type.
For example, even if
.Xr rgblink 1 Ap s
.Fl w
flag is used,
.Ic WRAM0
and
.Ic WRAMX
types are still considered different.
.It
Different constraints (alignment, bank, etc.) can be specified for each unionized section declaration, but they must all be compatible.
For example, alignment must be compatible with any fixed address, all specified banks must be the same, etc.
.It
Unionized sections cannot have type
.Ic ROM0
or
.Ic ROMX .
.El
.Pp
Different declarations of the same unionized section are not appended, but instead overlaid on top of eachother, just like
.Sx Unions .
Similarly, the size of an unionized section is the largest of all its declarations.
.Ss Section Fragments
Section fragments are sections with a small twist: when several of the same name are encountered, they are concatenated instead of producing an error.
This works within the same file (paralleling the behavior "plain" sections has in previous versions), but also across object files.
To declare an section fragment, add a
.Ic FRAGMENT
keyword after the
.Ic SECTION
one; the declaration is otherwise not different.
However, similarly to
.Sx Unionized Sections ,
some rules must be followed:
.Bl -bullet -offset indent
.It
If one section has been declared as fragment, all sections with the same name must be declared fragments as well.
.It
All declarations must have the same type.
For example, even if
.Xr rgblink 1 Ap s
.Fl w
flag is used,
.Ic WRAM0
and
.Ic WRAMX
types are still considered different.
.It
Different constraints (alignment, bank, etc.) can be specified for each unionized section declaration, but they must all be compatible.
For example, alignment must be compatible with any fixed address, all specified banks must be the same, etc.
.It
A section fragment may not be unionized; after all, that wouldn't make much sense.
.El
.Pp
When RGBASM merges two fragments, the one encountered later is appended to the one encountered earlier.
.Pp
When RGBLINK merges two fragments, the one whose file was specified last is appended to the one whose file was specified first.
For example, assuming
.Ql bar.o ,
.Ql baz.o ,
and
.Ql foo.o
all contain a fragment with the same name, the command
.Dl rgblink -o rom.gb baz.o foo.o bar.o
would produce the fragment from
.Ql baz.o
first, followed by the one from
.Ql foo.o ,
and the one from
.Ql bar.o
last.
.Sh SYMBOLS
RGBDS supports several types of symbols:
.Bl -hang
.It Sy Label
Numerical symbol designating a memory location.
May or may not have a value known at assembly time.
.It Sy Constant
Numerical symbol whose value has to be known at assembly time.
.It Sy Macro
A block of
.Nm
code that can be invoked later.
.It Sy String equate
String symbol that can be evaluated, similarly to a macro.
.El
.Pp
Symbol names can contain letters, numbers, underscores
.Sq _ ,
hashes
.Sq #
and at signs
.Sq @ .
However, they must begin with either a letter, or an underscore.
Periods
.Sq \&.
are allowed exclusively for labels, as described below.
A symbol cannot have the same name as a reserved keyword.
.Em \&In the line where a symbol is defined there must not be any whitespace before it ,
otherwise
.Nm
will treat it as a macro invocation.
.Bl -tag -width indent
.It Sy Label declaration
One of the assembler's main tasks is to keep track of addresses for you, so you can work with meaningful names instead of "magic" numbers.
.Pp
This can be done in a number of ways:
.Bd -literal -offset indent
GlobalLabel:
AnotherGlobal:
\&.locallabel
\&.yet_a_local:
AnotherGlobal.with_another_local:
ThisWillBeExported:: ;\ Note the two colons
ThisWillBeExported.too::
.Ed
.Pp
Declaring a label (global or local) with
.Ql ::
does an
.Ic EXPORT
at the same time.
(See
.Sx Exporting and importing symbols
below).
.Pp
Any label whose name does not contain a period is a global label, others are locals.
Declaring a global label sets it as the current label scope until the next one; any local label whose first character is a period will have the global label's name implicitly prepended.
Local labels can be declared as
.Ql scope.local:
or simply as as
.Ql .local: .
If the former notation is used, then
.Ql scope
must be the actual current scope.
.Pp
Local labels may have whitespace before their declaration as the only exception to the rule.
.Pp
.Sy Anonymous labels
are useful for short blocks of code.
They are defined like normal labels, but without a name before the colon.
Defining one does not change the label scope (unlike global labels).
Referencing one is done using a colon
.Ql \&:
followed by pluses
.Ql +
or minuses
.Ql - .
.Ic :+
references the next one after the expression,
.Ic :++
the one after it, and so on.
The logic is similar for -, just backwards.
.Bd -literal -offset indent
    ld hl, :++
:   ld a, [hli] ; Jumps to here
    ldh [c], a
    dec c
    jr nz, :-
    ret

: ; This address referenced by "ld hl"
    dw $7FFF, $1061, $03E0, $58A5
.Ed
.Pp
A label's location (and thus value) is usually not determined until the linking stage, so labels usually cannot be used as constants.
However, if the section in which the label is declared has a fixed base address, its value is known at assembly time.
.Pp
.Nm
is able to compute the subtraction of two labels either if both are constant as described above, or if both belong to the same section.
.It Ic EQU
.Ic EQU
allows defining constant symbols.
Unlike
.Ic SET
below, constants defined this way cannot be redefined.
They can, for example, be used for things such as bit definitions of hardware registers.
.Bd -literal -offset indent
SCREEN_WIDTH   equ 160 ;\ In pixels
SCREEN_HEIGHT  equ 144
.Ed
.Pp
Note that colons
.Ql \&:
following the name are not allowed.
.It Ic SET
.Ic SET ,
or its synonym
.Ic = ,
defines constant symbols like
.Ic EQU ,
but those constants can be redefined.
This is useful for variables in macros, for counters, etc.
.Bd -literal -offset indent
ARRAY_SIZE EQU 4
COUNT      SET 2
COUNT      SET ARRAY_SIZE+COUNT
;\ COUNT now has the value 6
COUNT      = COUNT + 1
.Ed
.Pp
Note that colons
.Ql \&:
following the name are not allowed.
.It Ic RSSET , RSRESET , RB , RW
The RS group of commands is a handy way of defining structures:
.Bd -literal -offset indent
              RSRESET
str_pStuff    RW   1
str_tData     RB   256
str_bCount    RB   1
str_SIZEOF    RB   0
.Ed
.Pp
The example defines four constants as if by:
.Bd -literal -offset indent
str_pStuff EQU 0
str_tData  EQU 2
str_bCount EQU 258
str_SIZEOF EQU 259
.Ed
.Pp
There are five commands in the RS group of commands:
.Bl -column "RSSET constexpr"
.It Sy Command Ta Sy Meaning
.It Ic RSRESET Ta Equivalent to Ql RSSET 0 .
.It Ic RSSET Ar constexpr Ta Sets the Ic _RS No counter to Ar constexpr .
.It Ic RB Ar constexpr Ta Sets the preceding symbol to Ic _RS No and adds Ar constexpr No to Ic _RS .
.It Ic RW Ar constexpr Ta Sets the preceding symbol to Ic _RS No and adds Ar constexpr No * 2 to Ic _RS .
.It Ic RL Ar constexpr Ta Sets the preceding symbol to Ic _RS No and adds Ar constexpr No * 4 to Ic _RS .
.El
.Pp
If the argument to
.Ic RB , RW ,
or
.Ic RL
is omitted, it's assumed to be 1.
.Pp
Note that colons
.Ql \&:
following the name are not allowed.
.It Ic EQUS
.Ic EQUS
is used to define string symbols.
Wherever the assembler meets a string symbol its name is replaced with its value.
If you are familiar with C you can think of it as similar to
.Fd #define .
.Bd -literal -offset indent
COUNTREG EQUS "[hl+]"
    ld a,COUNTREG

PLAYER_NAME EQUS "\[rs]"John\[rs]""
    db PLAYER_NAME
.Ed
.Pp
This will be interpreted as:
.Bd -literal -offset indent
    ld a,[hl+]
    db "John"
.Ed
.Pp
String symbols can also be used to define small one-line macros:
.Bd -literal -offset indent
pusha EQUS "push af\[rs]npush bc\[rs]npush de\[rs]npush hl\[rs]n"
.Ed
.Pp
Note that colons
.Ql \&:
following the name are not allowed.
.Pp
String equates can't be exported or imported.
.Pp
String equates, like
.Ic EQU
constants, cannot be redefined.
However, the
.Ic REDEF
keyword will define or redefine a string symbol.
For example:
.Bd -literal -offset indent
s EQUS "Hello, "
REDEF s EQUS "{s}world!"
; prints "Hello, world!"
PRINTT "{s}\n"
.Ed
.Pp
.Sy Important note :
An
.Ic EQUS
can be expanded to a string that contains another
.Ic EQUS
and it will be expanded as well.
If this creates an infinite loop,
.Nm
will error out once a certain depth is
reached.
See the
.Fl r
command-line option in
.Xr rgbasm 1 .
Also, a macro can contain an
.Ic EQUS
which calls the same macro, which causes the same problem.
.It Ic MACRO
One of the best features of an assembler is the ability to write macros for it.
Macros can be called with arguments, and can react depending on input using
.Ic IF
constructs.
.Bd -literal -offset indent
MyMacro: MACRO
         ld   a,80
         call MyFunc
         ENDM
.Ed
.Pp
Note that a single colon
.Ql \&:
following the macro's name is required.
Macros can't be exported or imported.
.Pp
Plainly nesting macro definitions is not allowed, but this can be worked around using
.Ic EQUS .
This won't work:
.Bd -literal -offset indent
outer: MACRO
inner: MACRO
    PRINTLN "Hello!"
ENDM
ENDM
.Ed
.Pp
But this will:
.Bd -literal -offset indent
outer: MACRO
definition equs "inner: MACRO\[rs]nPRINTLN \[rs]"Hello!\[rs]"\[rs]nENDM"
definition
    PURGE definition
ENDM
.Ed
.El
.Pp
Macro arguments support all the escape sequences of strings, as well as
.Ql \[rs],
to escape commas, since those otherwise separate arguments.
.Ss Exporting and importing symbols
Importing and exporting of symbols is a feature that is very useful when your project spans many source files and, for example, you need to jump to a routine defined in another file.
.Pp
Exporting of symbols has to be done manually, importing is done automatically if
.Nm
finds a symbol it does not know about.
.Pp
The following will cause
.Ar symbol1 , symbol2
and so on to be accessible to other files during the link process:
.Dl Ic EXPORT Ar symbol1 Bq , Ar symbol2 , No ...
.Pp
For example, if you have the following three files:
.Pp
.Ql a.asm :
.Bd -literal -compact
SECTION "a", WRAM0
LabelA:
.Ed
.Pp
.Ql b.asm :
.Bd -literal -compact
SECTION "b", WRAM0
ExportedLabelB1::
ExportedLabelB2:
	EXPORT ExportedLabelB2
.Ed
.Pp
.Ql c.asm :
.Bd -literal -compact
SECTION "C", ROM0[0]
	dw LabelA
	dw ExportedLabelB1
	dw ExportedLabelB2
.Ed
.Pp
Then
.Ql c.asm
can use
.Ql ExportedLabelB1
and
.Ql ExportedLabelB2 ,
but not
.Ql LabelA ,
so linking them together will fail:
.Bd -literal
$ rgbasm -o a.o a.asm
$ rgbasm -o b.o b.asm
$ rgbasm -o c.o c.asm
$ rgblink a.o b.o c.o
error: c.asm(2): Unknown symbol "LabelA"
Linking failed with 1 error
.Ed
.Pp
Note also that only exported symbols will appear in symbol and map files produced by
.Xr rgblink 1 .
.Pp
.Ss Purging symbols
.Ic PURGE
allows you to completely remove a symbol from the symbol table as if it had never existed.
.Em USE WITH EXTREME CAUTION!!!
I can't stress this enough,
.Sy you seriously need to know what you are doing .
DON'T purge a symbol that you use in expressions the linker needs to calculate.
When not sure, it's probably not safe to purge anything other than string symbols, macros, and constants.
.Bd -literal -offset indent
Kamikaze EQUS  "I don't want to live anymore"
AOLer    EQUS  "Me too"
         PURGE Kamikaze, AOLer
.Ed
.Pp
Note that, as an exception, string symbols in the argument list of a
.Ic PURGE
command
.Em will not be expanded .
.Ss Predeclared Symbols
The following symbols are defined by the assembler:
.Bl -column -offset indent "EQUS" "__ISO_8601_LOCAL__"
.It Sy Type Ta Sy Name Ta Sy Contents
.It Ic EQU Ta Dv @ Ta PC value (essentially, the current memory address)
.It Ic SET Ta Dv _RS Ta _RS Counter
.It Ic EQU Ta Dv _NARG Ta Number of arguments passed to macro, updated by Ic SHIFT
.It Ic EQU Ta Dv __LINE__ Ta The current line number
.It Ic EQUS Ta Dv __FILE__ Ta The current filename
.It Ic EQUS Ta Dv __DATE__ Ta Today's date
.It Ic EQUS Ta Dv __TIME__ Ta The current time
.It Ic EQUS Ta Dv __ISO_8601_LOCAL__ Ta ISO 8601 timestamp (local)
.It Ic EQUS Ta Dv __ISO_8601_UTC__ Ta ISO 8601 timestamp (UTC)
.It Ic EQU Ta Dv __UTC_YEAR__ Ta Today's year
.It Ic EQU Ta Dv __UTC_MONTH__ Ta Today's month number, 1\[en]12
.It Ic EQU Ta Dv __UTC_DAY__ Ta Today's day of the month, 1\[en]31
.It Ic EQU Ta Dv __UTC_HOUR__ Ta Current hour, 0\[en]23
.It Ic EQU Ta Dv __UTC_MINUTE__ Ta Current minute, 0\[en]59
.It Ic EQU Ta Dv __UTC_SECOND__ Ta Current second, 0\[en]59
.It Ic EQU Ta Dv __RGBDS_MAJOR__ Ta Major version number of RGBDS
.It Ic EQU Ta Dv __RGBDS_MINOR__ Ta Minor version number of RGBDS
.It Ic EQU Ta Dv __RGBDS_PATCH__ Ta Patch version number of RGBDS
.El
.Pp
The current time values will be taken from the
.Dv SOURCE_DATE_EPOCH
environment variable if that is defined as a UNIX timestamp.
Refer to the spec at
.Lk https://reproducible-builds.org/docs/source-date-epoch/ .
.Sh DEFINING DATA
.Ss Declaring variables in a RAM section
.Ic DS
allocates a number of empty bytes.
This is the preferred method of allocating space in a RAM section.
You can also use
.Ic DB , DW
and
.Ic DL
without any arguments instead (see
.Sx Defining constant data
below).
.Bd -literal -offset indent
DS 42 ;\ Allocates 42 bytes
.Ed
.Pp
Empty space in RAM sections will not be initialized.
In ROM sections, it will be filled with the value passed to the
.Fl p
command-line option, except when using overlays with
.Fl O .
.Ss Defining constant data
.Ic DB
defines a list of bytes that will be stored in the final image.
Ideal for tables and text.
.Bd -literal -offset indent
DB 1,2,3,4,"This is a string"
.Ed
.Pp
Alternatively, you can use
.Ic DW
to store a list of words (16-bit) or
.Ic DL
to store a list of double-words/longs (32-bit).
.Pp
Strings are handled a little specially: they first undergo charmap conversion (see
.Sx Character maps ) ,
then each resulting character is output individually.
For example, under the default charmap, the following two lines are identical:
.Bd -literal -offset indent
DW "Hello!"
DW "H", "e", "l", "l", "o", "!"
.Ed
.Pp
If you do not want this special handling, enclose the string in parentheses.
.Pp
.Ic DS
can also be used to fill a region of memory with some repeated values.
For example:
.Bd -literal -offset indent
; outputs 3 bytes: $AA, $AA, $AA
DS 3, $AA
; outputs 7 bytes: $BB, $CC, $BB, $CC, $BB, $CC, $BB
DS 7, $BB, $CC
.Ed
.Pp
You can also use
.Ic DB , DW
and
.Ic DL
without arguments, or leaving empty elements at any point in the list.
This works exactly like
.Ic DS 1 , DS 2
and
.Ic DS 4
respectively.
Consequently, no-argument
.Ic DB , DW
and
.Ic DL
can be used in a
.Ic WRAM0
/
.Ic WRAMX
/
.Ic HRAM
/
.Ic VRAM
/
.Ic SRAM
section.
.Ss Including binary files
You probably have some graphics, level data, etc. you'd like to include.
Use
.Ic INCBIN
to include a raw binary file as it is.
If the file isn't found in the current directory, the include-path list passed to
.Xr rgbasm 1
(see the
.Fl i
option) on the command line will be searched.
.Bd -literal -offset indent
INCBIN "titlepic.bin"
INCBIN "sprites/hero.bin"
.Ed
.Pp
You can also include only part of a file with
.Ic INCBIN .
The example below includes 256 bytes from data.bin, starting from byte 78.
.Bd -literal -offset indent
INCBIN "data.bin",78,256
.Ed
.Pp
The length argument is optional.
If only the start position is specified, the bytes from the start position until the end of the file will be included.
.Ss Unions
Unions allow multiple memory allocations to overlap, like unions in C.
This does not increase the amount of memory available, but allows re-using the same memory region for different purposes.
.Pp
A union starts with a
.Ic UNION
keyword, and ends at the corresponding
.Ic ENDU
keyword.
.Ic NEXTU
separates each block of allocations, and you may use it as many times within a union as necessary.
.Bd -literal -offset indent
    ; Let's say PC = $C0DE here
    UNION
    ; Here, PC = $C0DE
Name: ds 8
    ; PC = $C0E6
Nickname: ds 8
    ; PC = $C0EE
    NEXTU
    ; PC is back to $C0DE
Health: dw
    ; PC = $C0E0
Something: ds 6
    ; And so on
Lives: db
    NEXTU
VideoBuffer: ds 19
    ENDU
.Ed
.Pp
In the example above,
.Sq Name , Health , VideoBuffer
all have the same value, as do
.Sq Nickname
and
.Sq Lives .
Thus, keep in mind that
.Ic ld [Health], a
is identical to
.Ic ld [Name], a .
.Pp
The size of this union is 19 bytes, as this is the size of the largest block (the last one, containing
.Sq VideoBuffer ) .
Nesting unions is possible, with each inner union's size being considered as described above.
.Pp
Unions may be used in any section, but inside them may only be
.Ic DS -
like commands (see
.Sx Declaring variables in a RAM section ) .
.Sh THE MACRO LANGUAGE
.Ss Invoking macros
You execute the macro by inserting its name.
.Bd -literal -offset indent
         add a,b
         ld sp,hl
         MyMacro ;\ This will be expanded
         sub a,87
.Ed
.Pp
It's valid to call a macro from a macro (yes, even the same one).
.Pp
When
.Nm
sees
.Ic MyMacro
it will insert the macro definition (the code enclosed in
.Ic MACRO
/
.Ic ENDM ) .
.Pp
Suppose your macro contains a loop.
.Bd -literal -offset indent
LoopyMacro: MACRO
            xor  a,a
\&.loop       ld   [hl+],a
            dec  c
            jr   nz,.loop
ENDM
.Ed
.Pp
This is fine, but only if you use the macro no more than once per scope.
To get around this problem, there is the escape sequence
.Ic \[rs]@
that expands to a unique string.
.Pp
.Ic \[rs]@
also works in
.Ic REPT
blocks.
.Bd -literal -offset indent
LoopyMacro: MACRO
            xor  a,a
\&.loop\[rs]@     ld   [hl+],a
            dec  c
            jr   nz,.loop\[rs]@
ENDM
.Ed
.Pp
.Sy Important note :
Since a macro can call itself (or a different macro that calls the first one), there can be circular dependency problems.
If this creates an infinite loop,
.Nm
will error out once a certain depth is
reached.
See the
.Fl r
command-line option in
.Xr rgbasm 1 .
Also, a macro can have inside an
.Sy EQUS
which references the same macro, which has the same problem.
.Pp
It's possible to pass arguments to macros as well!
You retrieve the arguments by using the escape sequences
.Ic \[rs]1
through
.Ic \[rs]9 , \[rs]1
being the first argument specified on the macro invocation.
.Bd -literal -offset indent
LoopyMacro: MACRO
            ld   hl,\[rs]1
            ld   c,\[rs]2
            xor  a,a
\&.loop\[rs]@     ld   [hl+],a
            dec  c
            jr   nz,.loop\[rs]@
            ENDM
.Ed
.Pp
Now I can call the macro specifying two arguments, the first being the address and the second being a byte count.
The generated code will then reset all bytes in this range.
.Bd -literal -offset indent
LoopyMacro MyVars,54
.Ed
.Pp
Arguments are passed as string equates, although there's no need to enclose them in quotes.
Thus, an expression will not be evaluated first but kind of copy-pasted.
This means that it's probably a very good idea to use brackets around
.Ic \[rs]1
to
.Ic \[rs]9
if you perform further calculations on them.
For instance, consider the following:
.Bd -literal -offset indent
print_double: MACRO
    PRINTLN \[rs]1 * 2
ENDM
    print_double 1 + 2
.Ed
.Pp
The
.Ic PRINTLN
statement will expand to
.Ql PRINTLN 1 + 2 * 2 ,
which will print 5 and not 6 as you might have expected.
.Pp
Line continuations work as usual inside macros or lists of macro arguments.
However, some characters need to be escaped, as in the following example:
.Bd -literal -offset indent
PrintMacro: MACRO
    PRINT \[rs]1
ENDM

    PrintMacro STRCAT("Hello "\[rs], \[rs]
                      "world\[rs]n")
.Ed
.Pp
The comma needs to be escaped to avoid it being treated as separating the macro's arguments.
The backslash in
.Ql \[rs]n
does not need to be escaped because string literals also work as usual inside macro arguments.
.Pp
In reality, up to 256 arguments can be passed to a macro, but you can only use the first 9 like this.
If you want to use the rest, you need to use the
.Ic SHIFT
command.
.Pp
.Ic SHIFT
is a special command only available in macros.
Very useful in
.Ic REPT
blocks.
It will shift the arguments by one to the left, and decrease
.Dv _NARG
by 1.
.Ic \[rs]1
will get the value of
.Ic \[rs]2 , \[rs]2
will get the value of
.Ic \[rs]3 ,
and so forth.
.Pp
This is the only way of accessing the value of arguments from 10 to 256.
.Pp
.Ic SHIFT
can optionally be given an integer parameter, and will apply the above shifting that number of times.
A negative parameter will shift the arguments in reverse.
.Ss Printing things during assembly
The
.Ic PRINT
and
.Ic PRINTLN
commands print text and values to the standard output.
Useful for debugging macros, or wherever you may feel the need to tell yourself some important information.
.Bd -literal -offset indent
PRINT "Hello world!\[rs]n"
PRINTLN "Hello world!"
PRINT _NARG, " arguments\[rs]n"
PRINTLN "sum: ", 2+3, " product: ", 2*3
PRINTLN "Line #", __LINE__
PRINTLN STRFMT("E = %f", 2.718)
.Ed
.Bl -inset
.It Ic PRINT
prints out each of its comma-separated arguments.
Numbers are printed as unsigned uppercase hexadecimal with a leading
.Ic $ .
For different formats, use
.Ic STRFMT .
.It Ic PRINTLN
prints out each of its comma-separated arguments, if any, followed by a line feed
.Pq Ql \[rs]n .
.El
.Ss Automatically repeating blocks of code
Suppose you want to unroll a time consuming loop without copy-pasting it.
.Ic REPT
is here for that purpose.
Everything between
.Ic REPT
and the matching
.Ic ENDR
will be repeated a number of times just as if you had done a copy/paste operation yourself.
The following example will assemble
.Ql add a,c
four times:
.Bd -literal -offset indent
REPT 4
  add  a,c
ENDR
.Ed
.Pp
You can also use
.Ic REPT
to generate tables on the fly:
.Bd -literal -offset indent
; Generate a 256-byte sine table with values in the range [0, 128]
; (shifted and scaled from the range [-1.0, 1.0])
ANGLE = 0.0
    REPT 256
        db (MUL(64.0, SIN(ANGLE)) + 64.0) >> 16
ANGLE = ANGLE + 256.0 ; 256.0 = 65536 degrees / 256 entries
    ENDR
.Ed
.Pp
As in macros, you can also use the escape sequence
.Ic \[rs]@ .
.Ic REPT
blocks can be nested.
.Pp
A common pattern is to repeat a block for each value in some range.
.Ic FOR
is simpler than
.Ic REPT
for that purpose.
Everything between
.Ic FOR
and the matching
.Ic ENDR
will be repeated for each value of a given symbol.
For example, this code will produce a table of squared values from 0 to 255:
.Bd -literal -offset indent
FOR N, 256
      dw N * N
ENDR
.Ed
.Pp
It acts just as if you had done:
.Bd -literal -offset ident
N = 0
      dw N * N
N = 1
      dw N * N
N = 2
      dw N * N
; ...
N = 255
      dw N * N
N = 256
.Ed
.Pp
You can customize the range of
.Ic FOR
values:
.Bl -column "FOR V, start, stop, step"
.It Sy Code Ta Sy Range
.It Ic FOR Ar V , stop Ta Ar V No increments from 0 to Ar stop No
.It Ic FOR Ar V , start , stop Ta Ar V No increments from Ar start No to Ar stop No
.It Ic FOR Ar V , start , stop , step Ta Ar V No goes from Ar start No to Ar stop No by Ar step No
.El
.Pp
The
.Ic FOR
value will be updated by
.Ar step
until it reaches or exceeds
.Ar stop.
For example:
.Bd -literal -offset indent
FOR V, 4, 25, 5
      PRINT "{d:V} "
ENDR
      PRINTLN "done {d:V}"
.Ed
This will print:
.Bd -literal -offset indent
4 9 14 19 24 done 29
.Ed
.Pp
Just like with
.Ic REPT
blocks, you can use the escape sequence
.Ic \[rs]@
inside of
.Ic FOR
blocks, and they can be nested.
.Pp
You can stop a repeating block with the
.Ic BREAK
command.
A
.Ic BREAK
inside of a
.Ic REPT
or
.Ic FOR
block will interrupt the current iteration and not repeat any more.
It will continue running code after the block's
.Ic ENDR .
For example:
.Bd -literal -offset indent
FOR V, 1, 100
      PRINT "{d:V}"
      IF V == 5
          PRINT " stop! "
          BREAK
      ENDC
      PRINT ", "
ENDR
      PRINTLN "done {d:V}"
.Ed
This will print:
.Bd -literal -offset indent
1, 2, 3, 4, 5 stop! done 5
.Ed
.Ss Aborting the assembly process
.Ic FAIL
and
.Ic WARN
can be used to print errors and warnings respectively during the assembly process.
This is especially useful for macros that get an invalid argument.
.Ic FAIL
and
.Ic WARN
take a string as the only argument and they will print this string out as a normal error with a line number.
.Pp
.Ic FAIL
stops assembling immediately while
.Ic WARN
shows the message but continues afterwards.
.Pp
If you need to ensure some assumption is correct when compiling, you can use
.Ic ASSERT
and
.Ic STATIC_ASSERT .
Syntax examples are given below:
.Bd -literal -offset indent
Function:
      xor a
ASSERT LOW(Variable) == 0
      ld h, HIGH(Variable)
      ld l, a
      ld a, [hli]
      ; You can also indent this!
      ASSERT BANK(OtherFunction) == BANK(Function)
      call OtherFunction
; Lowercase also works
assert Variable + 1 == OtherVariable
      ld c, [hl]
      ret
\&.end
      ; If you specify one, a message will be printed
      STATIC_ASSERT .end - Function < 256, "Function is too large!"
.Ed
.Pp
First, the difference between
.Ic ASSERT
and
.Ic STATIC_ASSERT
is that the former is evaluated by RGBASM if it can, otherwise by RGBLINK; but the latter is only ever evaluated by RGBASM.
If RGBASM cannot compute the value of the argument to
.Ic STATIC_ASSERT ,
it will produce an error.
.Pp
Second, as shown above, a string can be optionally added at the end, to give insight into what the assertion is checking.
.Pp
Finally, you can add one of
.Ic WARN , FAIL
or
.Ic FATAL
as the first optional argument to either
.Ic ASSERT
or
.Ic STATIC_ASSERT .
If the assertion fails,
.Ic WARN
will cause a simple warning (controlled by
.Xr rgbasm 1
flag
.Fl Wassert )
to be emitted;
.Ic FAIL
(the default) will cause a non-fatal error; and
.Ic FATAL
immediately aborts.
.Ss Including other source files
Use
.Ic INCLUDE
to process another assembler file and then return to the current file when done.
If the file isn't found in the current directory, the include path list (see the
.Fl i
option in
.Xr rgbasm 1 )
will be searched.
You may nest
.Ic INCLUDE
calls infinitely (or until you run out of memory, whichever comes first).
.Bd -literal -offset indent
    INCLUDE "irq.inc"
.Ed
.Ss Conditional assembling
The four commands
.Ic IF , ELIF , ELSE ,
and
.Ic ENDC
let you have
.Nm
skip over parts of your code depending on a condition.
This is a powerful feature commonly used in macros.
.Bd -literal -offset indent
IF NUM < 0
  PRINTLN "NUM < 0"
ELIF NUM == 0
  PRINTLN "NUM == 0"
ELSE
  PRINTLN "NUM > 0"
ENDC
.Ed
.Pp
The
.Ic ELIF
(standing for "else if") and
.Ic ELSE
blocks are optional.
.Ic IF
/
.Ic ELIF
/
.Ic ELSE
/
.Ic ENDC
blocks can be nested.
.Pp
Note that if an
.Ic ELSE
block is found before an
.Ic ELIF
block, the
.Ic ELIF
block will be ignored.
All
.Ic ELIF
blocks must go before the
.Ic ELSE
block.
Also, if there is more than one
.Ic ELSE
block, all of them but the first one are ignored.
.Sh MISCELLANEOUS
.Ss Changing options while assembling
.Ic OPT
can be used to change some of the options during assembling from within the source, instead of defining them on the command-line.
.Pp
.Ic OPT
takes a comma-separated list of options as its argument:
.Bd -literal -offset indent
PUSHO
OPT   g.oOX ;Set the GB graphics constants to use these characters
DW    `..ooOOXX
POPO
DW    `00112233
.Ed
.Pp
The options that OPT can modify are currently:
.Cm b , g
and
.Cm p .
.Pp
.Ic POPO
and
.Ic PUSHO
provide the interface to the option stack.
.Ic PUSHO
will push the current set of options on the option stack.
.Ic POPO
can then later be used to restore them.
Useful if you want to change some options in an include file and you don't want to destroy the options set by the program that included your file.
The stack's number of entries is limited only by the amount of memory in your machine.
.Ss Requesting alignment
While
.Ic ALIGN
as presented in
.Sx SECTIONS
is often useful as-is, sometimes you instead want a particular piece of data (or code) in the middle of the section to be aligned.
This is made easier through the use of mid-section
.Ic align Ar align , offset .
It will alter the section's attributes to ensure that the location the
.Ic align
directive is at, has its
.Ar align
lower bits equal to
.Ar offset .
.Pp
If the constraint cannot be met (for example because the section is fixed at an incompatible address), and error is produced.
Note that
.Ic align Ar align
is a shorthand for
.Ic align Ar align , No 0 .
.Sh SEE ALSO
.Xr rgbasm 1 ,
.Xr rgblink 1 ,
.Xr rgblink 5 ,
.Xr rgbds 5 ,
.Xr rgbds 7 ,
.Xr gbz80 7
.Sh HISTORY
.Nm
was originally written by Carsten S\(/orensen as part of the ASMotor package,
and was later packaged in RGBDS by Justin Lloyd.
It is now maintained by a number of contributors at
.Lk https://github.com/gbdev/rgbds .