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 March 13, 2018
.Dt RGBASM 5
.Os RGBDS Manual
.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 GameBoy CPU is in
.Xr gbz80 7 .
.Pp
.Sh GENERAL
.Ss Syntax
The syntax is line‐based, just as in any other assembler, meaning that you do
one instruction or pseudo‐op per line:
.Pp
.Dl Oo Ar label Oc Oo Ar instruction Oc Oo Ar \&;comment Oc
.Pp
Example:
.Pp
.Bd -literal -offset indent
John: ld a,87 ;Weee
.Ed
.Pp
All pseudo‐ops, mnemonics and registers (reserved keywords) are case‐insensitive
and all labels are case‐sensitive.
.Pp
There are two syntaxes for comments.
In both cases, a comment ends at the end of the line.
The most common one is: anything that follows a semicolon
.Ql \&;
(that isn't inside a string) is a comment.
There is another format: anything that follows a
.Ql *
that is placed right at the start of
a line is a comment.
The assembler removes all comments from the code before doing anything else.
.Pp
Sometimes lines can be too long and it may be necessary to split them.
The syntax to do so is the following one:
.Pp
.Bd -literal -offset indent
    DB 1, 2, 3, 4 \[rs]
       5, 6, 7, 8
.Ed
.Pp
This works anywhere in the code except inside of strings.
To split strings it is needed to use
.Fn STRCAT
like this:
.Pp
.Bd -literal -offset indent
    DB STRCAT("Hello ", \[rs]
              "world!")
.Ed
.Pp
.Ss Sections
.Ic SECTION Ar name , type
.Pp
.Ic SECTION Ar name , type , options
.Pp
.Ic SECTION Ar name , type Ns Bo Ar addr Bc
.Pp
.Ic SECTION Ar name , type Ns Bo Ar addr Bc , Ar options
.Pp
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
.Ar name
is a string enclosed in double quotes and can be a new name or the name of an
existing section.
All sections assembled at the same time that have the same name and type are
considered to be the same section, and their code is put together in the object
file generated by the assembler.
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:
.Pp
.Bl -tag
.It Cm ROM0
A ROM section.
.Ar addr
can range from $0000–$3FFF (or $0000–$7FFF if tiny ROM mode is enabled in
.Xr rgblink 1 ) .
.It Cm ROMX
A banked ROM section.
.Ar addr
can range from $4000–$7FFF.
.Ar bank
can range from 1 to 511.
Not available if tiny ROM mode is enabled in
.Xr rgblink 1 .
.It Cm VRAM
A banked video RAM section.
.Ar addr
can range from $8000–$9FFF.
.Ar bank
can be 0 or 1 but bank 1 is unavailable if DMG mode is enabled in
.Xr rgblink 1 .
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm SRAM
A banked external (save) RAM section.
.Ar addr
can range from $A000–$BFFF.
.Ar bank
can range from 0 to 15.
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm WRAM0
A general-purpose RAM section.
.Ar addr
can range from $C000–$CFFF, or $C000–$DFFF if DMG mode is enabled in
.Xr rgblink 1 .
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm WRAMX
A banked general-purpose RAM section.
.Ar addr
can range from $D000–$DFFF.
.Ar bank
can range from 1 to 7.
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
Not available if DMG mode is enabled in
.Xr rgblink 1 .
.It Cm OAM
An object attributes RAM section.
.Ar addr
can range from $FE00-$FE9F.
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm HRAM
A high RAM section.
.Ar addr
can range from $FF80–$FFFE.
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.Pp
.Sy Note :
If you use this method of allocating HRAM the assembler will
.Em not
choose the short addressing mode in the LD instructions
.Sy LD [$FF00+n8],A
and
.Sy LD A,[$FF00+n8]
because the actual address calculation is done by the linker.
If you find this undesirable you can use
.Ic RSSET , RB ,
or
.Ic RW
instead or use the
.Sy LDH [$FF00+n8],A
and
.Sy LDH A,[$FF00+n8]
syntax instead.
This forces the assembler to emit the correct instruction and the linker to
check if the value is in the correct range.
This optimization can be disabled by passing the
.Fl L
flag to
.Xr rgbasm 1 .
.El
.Pp
.Ar option Ns s are comma separated and may include:
.Bl -tag
.It Cm BANK Ns Bq Ar bank
Specify which
.Ar bank
for the linker to place the section.
.It Cm ALIGN Ns Bq Ar align
Place the section at an address whose
.Ar align
least‐significant bits are zero.
It is a syntax error to use this option with
.Ar addr .
.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
.Cm 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 linkerscript 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:
.Bd -literal -offset indent
    SECTION "CoolStuff",ROMX
.Ed
.Pp
This switches to the section called
.Dq CoolStuff
(or creates it if it doesn't already exist) and defines it as a code section.
.Pp
The following example defines a section that can be placed anywhere in any ROMX
bank:
.Pp
.Bd -literal -offset indent
    SECTION "CoolStuff",ROMX
.Ed
.Pp
If it is needed, the the base address of the section can be specified:
.Pp
.Bd -literal -offset indent
    SECTION "CoolStuff",ROMX[$4567]
.Ed
.Pp
An example with a fixed bank:
.Pp
.Bd -literal -offset indent
    SECTION "CoolStuff",ROMX[$4567],BANK[3]
.Ed
.Pp
And if you only want to force the section into a certain bank, and not it's
position within the bank, that's also possible:
.Pp
.Bd -literal -offset indent
    SECTION "CoolStuff",ROMX,BANK[7]
.Ed
.Pp
Alignment examples:
one use could be when using DMA to copy data or when it is needed to align the
start of an array to 256 bytes to optimize the code that accesses it.
.Pp
.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
.Pp
.Sy Hint :
If you think this is a lot of typing for doing a simple
.Dq org
type thing you can quite easily write an intelligent macro (called
.Ic ORG
for example) that uses
.Ic @
for the section name and determines
correct section type etc as arguments for
.Ic SECTION .
.Ss Section Stack
.Ic POPS
and
.Ic PUSHS
provide the interface to the section stack.
.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 destroy
the section context for the program that included your file.
The number of entries in the stack is limited only by the amount of memory in
your machine.
.Sh SYMBOLS
.Pp
.Ss Symbols
RGBDS supports several types of symbols:
.Pp
.Bl -hang
.It Sy Label
Used to assign a memory location with a name
.It Sy EQUate
Give a constant a name.
.It Sy SET
Almost the same as EQUate, but you can change the value of a SET during
assembling.
.It Sy Structure Po Sy the RS group Pc
Define a structure easily.
.It Sy String equate Pq Sy EQUS
Give a frequently used string a name.
Can also be used as a mini-macro, like
.Fd #define
in C.
.It Sy MACRO
A block of code or pseudo instructions that you invoke like any other mnemonic.
You can give them arguments too.
.El
.Pp
A symbol cannot have the same name as a reserved keyword.
.Bl -hang
.It Sy Label
.Pp
One of the assembler's main tasks is to keep track of addresses for you so you
don't have to remember obscure numbers but can make do with a meaningful name, a
label.
.Pp
This can be done in a number of ways:
.Pp
.Bd -literal -offset indent
GlobalLabel
AnotherGlobal:
\&.locallabel
\&.yet_a_local:
AnotherGlobal.with_another_local:
ThisWillBeExported:: ;note the two colons
ThisWillBeExported.too::
.Ed
.Pp
In the line where a label is defined there musn't be any whitespace before it.
Local labels are only accessible within the scope they are defined.
A scope starts after a global label and ends at the next global label.
Declaring a label (global or local) with :: does an EXPORT at the same time.
Local labels can be declared as scope.local or simply as as .local.
If the former notation is used, the scope must be the actual current scope.
.Pp
Labels will normally change their value during the link process and are thus not
constant.
The exception is the case in which the base address of a section is fixed, so
the address of the label is known at assembly time.
.Pp
The subtraction of two labels is only constant (known at assembly time) if they
are two local labels that belong to the same scope, or they are two global
labels that belong to sections with fixed base addresses.
.Pp
.It Sy EQU
.Pp
EQUates are constant symbols.
They can, for example, be used for things such as bit-definitions of hardware
registers.
.Pp
.Bd -literal -offset indent
EXIT_OK      EQU $00
EXIT_FAILURE EQU $01
.Ed
.Pp
Note that a colon (:) following the label-name is not allowed.
EQUates cannot be exported and imported.
They don't change their value during the link process.
.It Sy SET
.Pp
SETs are similar to EQUates.
They are also constant symbols in the sense that their values are defined during
the assembly process.
These symbols are normally used in macros.
.Pp
.Bd -literal -offset indent
ARRAY_SIZE EQU 4
COUNT      SET 2
COUNT      SET ARRAY_SIZE+COUNT
.Ed
.Pp
Note that a colon (:) following the label-name is not allowed.
SETs cannot be exported and imported.
Alternatively you can use = as a synonym for SET.
.Pp
.Bd -literal -offset indent
COUNT = 2
.Ed
.Pp
.It Sy RSSET , RSRESET , RB , RW
.Pp
The RS group of commands is a handy way of defining structures:
.Pp
.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 equated symbols:
.Pp
.Bd -literal -offset indent
str_pStuff = 0
str_tData  = 2
str_bCount = 258
str_SIZEOF = 259
.Ed
.Pp
There are four commands in the RS group of commands:
.Pp
.Bl -column "RSSET constexpr"
.It Sy Command Ta Sy Meaning
.It Ic RSRESET Ta Resets the _RS counter to zero.
.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
Note that a colon (:) following the symbol-name is not allowed.
.Sy RS
symbols cannot be exported and imported.
They don't change their value during the link process.
.Pp
.It Sy EQUS
.Pp
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 the same as
.Fd #define .
.Pp
.Bd -literal -offset indent
COUNTREG EQUS "[hl+]"
    ld a,COUNTREG

PLAYER_NAME EQUS "\[rs]"John\[rs]""
    db PLAYER_NAME
.Ed
.Pp
Note that : following the label-name is not allowed, and that strings must be
quoted to be useful.
.Pp
This will be interpreted as:
.Pp
.Bd -literal -offset indent
    ld a,[hl+]
    db "John"
.Ed
.Pp
String-symbols can also be used to define small one-line macros:
.Pp
.Bd -literal -offset indent
PUSHA EQUS "push af\[rs]npush bc\[rs]npush de\[rs]npush hl\[rs]n"
.Ed
.Pp
Note that a colon (:) following the label-name is not allowed.
String equates can't be exported or imported.
.Pp
.Sy Important note :
An EQUS can be expanded to a string that contains another EQUS
and it will be expanded as well.
This means that, if you aren't careful, you may trap the assembler into an
infinite loop if there's a circular dependency in the expansions.
Also, a MACRO can have inside an EQUS which references the same MACRO, which has
the same problem.
.Pp
.It Sy MACRO
.Pp
One of the best features of an assembler is the ability to write macros for it.
Macros also provide a method of passing arguments to them and they can then
react to the input using IF-constructs.
.Pp
.Bd -literal -offset indent
MyMacro: MACRO
         ld   a,80
         call MyFunc
         ENDM
.Ed
.Pp
Note that a colon (:) following the macro-name is required.
Macros can't be exported or imported.
It's valid to call a macro from a macro (yes, even the same one).
.Pp
The above example is a very simple macro.
You execute the macro by typing its name.
.Pp
.Bd -literal -offset indent
         add  a,b
         ld   sp,hl
         MyMacro ;This will be expanded
         sub  a,87
.Ed
.Pp
When the assembler meets MyMacro it will insert the macrodefinition (the text
enclosed in
.Ic MACRO
/
.Ic ENDM ) .
.Pp
Suppose your macro contains a loop.
.Pp
.Bd -literal -offset indent
LoopyMacro: MACRO
            xor  a,a
\&.loop       ld   [hl+],a
            dec  c
            jr   nz,.loop
            ENDM
.Ed
.Pp
This is fine.
That is, if you only use the macro once per scope.
To get around this problem there is a special label string equate called
.Ic \[rs]@
that you can append to your labels and it will then expand to a unique string.
.Pp
.Ic \[rs]@
also works in REPT-blocks should you have any loops there.
.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 problems of circular dependency.
They trap the assembler in an infinite loop, so you have to be careful when
using recursion with MACROs.
Also, a MACRO can have inside an EQUS which references the same MACRO, which has
the same problem.
.Pp
.Sy Macro Arguments
.Pp
I'd like LoopyMacro a lot better if I didn't have to pre-load the registers
with values and then call it.
What I'd like is the ability to pass it arguments and it then loaded the
registers itself.
.Pp
And I can do that.
In macros you can get the arguments by using the special macro string equates
.Ic \[rs]1
through
.Ic \[rs]9 ,
.Ic \[rs]1
being the first argument
specified on the calling of the macro.
.Pp
.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 bytecount.
The macro will then reset all bytes in this range.
.Pp
.Bd -literal -offset indent
LoopyMacro MyVars,54
.Ed
.Pp
Arguments are passed as string equates.
There's no need to enclose them in quotes.
An expression will not be evaluated first but passed directly.
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, if you pass 1 + 2 as the first argument and then do
.Ic PRINTV
.Ic \[rs]1
* 2
you will get the value 5 on screen and not 6 as you might have expected.
.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 keyword
.Ic SHIFT .
.Pp
Line continuations work as usual inside macros or lists of arguments of macros.
Strings, however, are a bit trickier.
The following example shows how to use strings as arguments for a macro:
.Pp
.Bd -literal -offset indent
PrintMacro : MACRO
    PRINTT \[rs]1
ENDM

    PrintMacro STRCAT(\[rs]"Hello\[rs]"\[rs],  \[rs]
                      \[rs]" world\[rs]\[rs]n\[rs]")
.Ed
.Pp
.Ic SHIFT
is a special command only available in macros.
Very useful in REPT-blocks.
It will shift the arguments by one to the left.
.Ic \[rs]1
will get the value of
.Ic \[rs]2 ,
.Ic \[rs]2
will get the value in
.Ic \[rs]3
and so forth.
.Pp
This is the only way of accessing the value of arguments from 10 to 256.
.Pp
.El
.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 the assembler doesn't know where a symbol is defined.
.Pp
.Ic EXPORT Ar label Bq , Ar label No , ...
.Pp
The assembler will make label accessible to other files during the link process.
.Pp
.Ic GLOBAL Ar label Bq , Ar label No , ...
.Pp
If label is defined during the assembly it will be exported, if not, it will be
imported.
Handy (very!) for include-files.
Note that, since importing is done automatically, this keyword has the same
effect as
.Ic EXPORT .
.Ss Purging symbols
.Ic PURGE
allows you to completely remove a symbol from the symbol table as if it had
never existed.
USE WITH EXTREME CAUTION!!!
I can't stress this enough, you seriously need to know what you are doing.
DON'T purge symbol that you use in expressions the linker needs to calculate.
In fact, it's probably not even safe to purge anything other than string symbols
and macros.
.Pp
.Bd -literal -offset indent
Kamikaze EQUS  "I don't want to live anymore"
AOLer    EQUS  "Me too"
         PURGE Kamikaze, AOLer
.Ed
.Pp
Note that string symbols that are part of a
.Ic PURGE
command WILL NOT BE EXPANDED as the ONLY exception to this rule.
.Ss Predeclared Symbols
The following symbols are defined by the assembler:
.Pp
.Bl -column -offset indent "EQUS" "__ISO_8601_LOCAL__"
.It Sy Type Ta Sy Name Ta Sy Contents
.It Ic EQU Ta Ic @ Ta PC value
.It Ic EQU Ta Ic _PI Ta Fixed point \[*p]
.It Ic SET Ta Ic _RS Ta _RS Counter
.It Ic EQU Ta Ic _NARG Ta Number of arguments passed to macro
.It Ic EQU Ta Ic __LINE__ Ta The current line number
.It Ic EQUS Ta Ic __FILE__ Ta The current filename
.It Ic EQUS Ta Ic __DATE__ Ta Today's date
.It Ic EQUS Ta Ic __TIME__ Ta The current time
.It Ic EQUS Ta Ic __ISO_8601_LOCAL__ Ta ISO 8601 timestamp (local)
.It Ic EQUS Ta Ic __ISO_8601_UTC__ Ta ISO 8601 timestamp (UTC)
.It Ic EQU Ta Ic __UTC_YEAR__ Ta Today's year
.It Ic EQU Ta Ic __UTC_MONTH__ Ta Today's month number, 1-12
.It Ic EQU Ta Ic __UTC_DAY__ Ta Today's day of the month, 1-31
.It Ic EQU Ta Ic __UTC_HOUR__ Ta Current hour, 0-23
.It Ic EQU Ta Ic __UTC_MINUTE__ Ta Current minute, 0-59
.It Ic EQU Ta Ic __UTC_SECOND__ Ta Current second, 0-59
.It Ic EQU Ta Ic __RGBDS_MAJOR__ Ta Major version number of RGBDS.
.It Ic EQU Ta Ic __RGBDS_MINOR__ Ta Minor version number of RGBDS.
.It Ic EQU Ta Ic __RGBDS_PATCH__ Ta Patch version number of RGBDS.
.El
.Pp
.Sh DEFINING DATA
.Ss Defining constant data
.Ic DB
defines a list of bytes that will be stored in the final image.
Ideal for tables and text (which is not zero-terminated).
.Pp
.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-bits) or
.Ic DL
to store a list of doublewords/longs (32-bits).
Strings are not allowed as arguments to
.Ic DW
and
.Ic DL .
.Pp
You can also use
.Ic DB ,
.Ic DW
and
.Ic DL
without arguments, or leaving empty elements at any point in the list.
This works exactly like
.Sy DS 1 ,
.Sy DS 2
and
.Sy DS 4
respectively.
Consequently,
.Ic DB ,
.Ic DW
and
.Ic DL
can be used in a
.Sy WRAM0 No / Sy WRAMX No / Sy HRAM No / Sy VRAM No / Sy SRAM
section.
.Ss Declaring variables in a RAM section
.Ic DS
allocates a number of bytes.
The content is undefined.
This is the preferred method of allocationg space in a RAM section.
You can, however, use
.Ic DB ,
.Ic DW
and
.Ic DL
without any arguments instead.
.Pp
.Bd -literal -offset indent
DS str_SIZEOF ;allocate str_SIZEOF bytes
.Ed
.Pp
.Ss Including binary files
You probably have some graphics 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 the linker on the command line will be searched.
.Pp
.Bd -literal -offset indent
INCBIN "titlepic.bin"
INCBIN "sprites/hero.bin"\ ; UNIX
INCBIN "sprites\[rs]\[rs]hero.bin"\ ; Windows
.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.
.Pp
.Bd -literal -offset indent
INCBIN "data.bin",78,256
.Ed
.Ss Unions
Unions allow multiple memory allocations to share the same space in memory,
like unions in C.
This allows you to easily reuse memory for different purposes, depending on
the game's state.
.Pp
You create unions using the
.Ic UNION ,
.Ic NEXTU
and
.Ic ENDU
keywords.
.Ic NEXTU
lets you create a new block of allocations, and you may use it as many times
within a union as necessary.
.Pp
.Bd -literal -offset indent
UNION
Name: ds 8
Nickname: ds 8
NEXTU
Health: dw
Something: ds 3
Lives: db
NEXTU
Temporary: ds 19
ENDU
.Ed
.Pp
This union will use up 19 bytes, as this is the size of the largest block
(the last one, containing 'Temporary').
Of course, as 'Name', 'Health', and 'Temporary' all point to the same memory
locations, writes to any one of these will affect values read from the others.
.Pp
Unions may be used in any section, but code and data may not be included.
.Sh THE MACRO LANGUAGE
.Pp
.Ss Printing things during assembly
These three instructions type text and values to stdout.
Useful for debugging macros or wherever you may feel the need to tell yourself
some important information.
.Pp
.Bd -literal -offset indent
PRINTT "I'm the greatest programmer in the whole wide world\[rs]n"
PRINTI (2 + 3) / 5
PRINTV $FF00 + $F0
PRINTF MUL(3.14, 3987.0)
.Ed
.Pp
.Bl -inset
.It Ic PRINTT
prints out a string.
.It Ic PRINTV
prints out an integer value in hexadecimal or, as in the example, the result of
a calculation.
Unsurprisingly, you can also print out a constant symbols value.
.It Ic PRINTI
prints out a signed integer value.
.It Ic PRINTF
prints out a fixed point value.
.El
.Ss Automatically repeating blocks of code
Suppose you're feeling lazy and you want to unroll a time consuming loop.
.Ic REPT
is here for that purpose.
Everything between
.Ic REPT
and
.Ic ENDR
will be repeated a number of times just as if you done a copy/paste operation
yourself.
The following example will assemble
.Sy add a,c
four times:
.Pp
.Bd -literal -offset indent
REPT 4
add  a,c
ENDR
.Ed
.Pp
You can also use
.Ic REPT
to generate tables on the fly:
.Pp
.Bd -literal -offset indent
; --
; -- Generate a 256 byte sine table with values between 0 and 128
; --
ANGLE SET   0.0
      REPT  256
      DB    (MUL(64.0,SIN(ANGLE))+64.0)>>16
ANGLE SET ANGLE+256.0
      ENDR
.Ed
.Pp
.Ic REPT
is also very useful in recursive macros and, as in macros, you can also use the
special label operator
.Ic \[rs]@ .
REPT-blocks can be nested.
.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.
.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 will be
searched.
You may nest
.Ic INCLUDE
calls infinitely (or until you run out of memory, whichever comes first).
.Pp
.Bd -literal -offset indent
    INCLUDE "irq.inc"
.Ed
.Pp
.Ss Conditional assembling
The four commands
.Ic IF ,
.Ic ELIF ,
.Ic ELSE ,
and
.Ic ENDC
are used to conditionally assemble parts of your file.
This is a powerful feature commonly used in macros.
.Pp
.Bd -literal -offset indent
IF NUM < 0
  PRINTT "NUM < 0\[rs]n"
ELIF NUM == 0
  PRINTT "NUM == 0\[rs]n"
ELSE
  PRINTT "NUM > 0\[rs]n"
ENDC
.Ed
.Pp
The
.Ic ELIF
and
.Ic ELSE
blocks are optional.
.Ic IF No / Ic ELIF No / Ic ELSE No / 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.
.Ss Integer and Boolean expressions
An expression can be composed of many things.
Expressions are always evaluated using signed 32-bit math.
.Pp
The most basic expression is just a single number.
.Pp
.Sy Numeric Formats
.Pp
There are a number of numeric formats.
.Pp
.Bl -dash -compact
.It
Hexadecimal: $0123456789ABCDEF.
Case-insensitive
.It
Decimal: 0123456789
.It
Octal: &01234567
.It
Binary: %01
.It
Fixedpoint (16.16): 01234.56789
.It
Character constant: "ABYZ"
.It
Gameboy graphics: \`0123
.El
.Pp
The last one, Gameboy graphics, is quite interesting and useful.
The values are actually pixel values and it converts the
.Do chunky Dc data to Do planar Dc data as used in the Gameboy.
.Pp
.Bd -literal -offset indent
    DW \`01012323
.Ed
.Pp
Admittedly, an expression with just a single number is quite boring.
To spice things up a bit there are a few operators you can use to perform
calculations between numbers.
.Pp
.Sy Operators
.Pp
A great number of operators you can use in expressions are available (listed in
order of precedence):
.Pp
.Bl -column -offset indent "Operator"
.It Sy Operator Ta Sy Meaning
.It Li \&( \&) Ta Precedence override
.It Li FUNC() Ta Function call
.It Li ~ + - Ta Unary not/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 Boolean comparison
.It Li >= < > Ta Boolean comparison (Same precedence as the others)
.It Li && || Ta Boolean and/or
.It Li \&! Ta Unary Boolean not
.El
.Pp
The result of the boolean operators is zero if when FALSE and non-zero when
TRUE.
It is legal to use an integer as the condition for IF blocks.
You can use symbols instead of numbers in your expression if you wish.
.Pp
An expression is said to be constant when it doesn't change its value during
linking.
This basically means that you can't use labels in those expressions.
The instructions in the macro-language all require expressions that are
constant.
The only exception is the subtraction of labels in the same section or labels
that belong to sections with a fixed base addresses, all of which must be
defined in the same source file (the calculation cannot be passed to the object
file generated by the assembler).
In this case, the result is a constant that can be calculated at assembly time.
.Pp
.Ss Fixed‐point Expressions
Fixed point constants are basically normal 32-bit constants where the upper 16
bits are used for the integer part and the lower 16 bits are used for the
fraction (65536ths).
This means that you can use them in normal integer expression, and some integer
operators like plus and minus don't care whether the operands are integer or
fixed-point.
You can easily convert a fixed-point number to an integer by shifting it right
16 bits.
It follows that you can convert an integer to a fixed-point number by shifting
it left.
.Pp
Some things are different for fixed-point math, though, which is why you have
the following functions to use:
.EQ
delim $$
.EN
.Pp
.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 SIN x Ta $sin ( x )$
.It Fn COS x Ta $cos ( x )$
.It Fn TAN x Ta $tan ( x )$
.It Fn ASIN x Ta $asin ( x )$
.It Fn ACOS x Ta $acos ( x )$
.It Fn ATAN x Ta $atan ( x )$
.It Fn ATAN2 x y Ta Angle between $( x , y )$ and $( 1 , 0 )$
.El
.EQ
delim off
.EN
.Pp
These functions are extremely useful for automatic generation of various tables.
A circle has 65536.0 degrees.
Sine values are between
.Bq -1.0 ; 1.0 .
.Pp
.Bd -literal -offset indent
; --
; -- Generate a 256 byte sine table with values between 0 and 128
; --
ANGLE SET   0.0
      REPT  256
      DB    (MUL(64.0,SIN(ANGLE))+64.0)>>16
ANGLE SET ANGLE+256.0
      ENDR
.Ed
.Pp
.Ss String Expressions
The most basic string expression is any number of characters contained in double
quotes ("for instance").
Like in C, the escape character is \[rs], and there are a number of commands you
can use within a string:
.Pp
.Bl -column -offset indent "String"
.It Sy String Ta Sy Meaning
.It Li \[rs]\[rs] Ta Backslash
.It Li \[rs]" Ta Double quote
.It Li \[rs], Ta Comma
.It Li \[rs]{ Ta Curly bracket left
.It Li \[rs]} Ta Curly bracket right
.It Li \[rs]n Ta Newline ($0A)
.It Li \[rs]t Ta Tab ($09)
.It Li \[rs]1 - \[rs]9 Ta Macro argument (Only the body of a macros)
.It Li \[rs]@ Ta Label name suffix (Only in the body of macros and repts)
.El
.Pp
A funky feature is
.Sy {symbol}
within a string.
This will examine the type of the symbol and insert its value accordingly.
If symbol is a string symbol, the symbols value is simply copied.
If it's a numeric symbol, the value is converted to hexadecimal notation and
inserted as a string.
.Pp
HINT: The
.Sy {symbol}
construct can also be used outside strings.
The symbol's value is again inserted as a string.
This is just a short way of doing
.Dq {symbol} .
.Pp
Whenever the macro-language expects a string you can actually use a string
expression.
This consists of one or more of these function (yes, you can nest them).
Note that some of these functions actually return an integer and can be used as
part of an integer expression!
.Pp
.Bl -column "STRSUB_str,_pos,_len"
.It Sy Name Ta Sy Operation
.It Fn STRLEN string  Ta Returns the number of characters in string
.It Fn STRCAT str1 str2  Ta Appends str2 to str1.
.It Fn STRCMP str1 str2  Ta Returns negative if str1 is alphabetically lower
than str2, zero if they match, positive if str1 is greater than str2.
.It Fn STRIN str1 str2  Ta Returns the position of str2 in str1 or zero if it's
not present (first character is position 1).
.It Fn STRSUB str pos len  Ta Returns a substring from str starting at pos
(first character is position 1) and with len characters.
.It Fn STRUPR str  Ta Converts all characters in str to capitals and returns the
new string.
.It Fn STRLWR str  Ta Converts all characters in str to lower case and returns
the new string.
.El
.Pp
.Ss Character maps
.Pp
When writing text that is meant to be displayed in the Game Boy, the ASCII
characters used in the source code may not be the same ones used in the tileset
used in the ROM.
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 the code to map strings up to 16 characters long to an
abitrary 8-bit value:
.Pp
.Bd -literal -offset indent
CHARMAP "<LF>", 10
CHARMAP "&iacute", 20
CHARMAP "A", 128
.Ed
.Pp
.Sy Note:
Character maps affect all strings in the file from the point in which they are
defined.
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.
.Pp
.Ss Other functions
There are a few other functions that do various useful things:
.Pp
.Bl -column "BANK(arg)"
.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.
For labels, as the linker has to resolve this, it can't be used when the
expression has to be constant.
.It Fn DEF label  Ta Returns TRUE if
.Ar label
has been defined.
.It Fn HIGH arg Ta Returns the top 8 bits of the operand if
.Ar arg
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
is a label or constant, or the bottom 8-bit register if it is a 16-bit register
(AF isn't a valid register for this function).
.El
.Pp
.Sh MISCELLANEOUS
.Ss Changing options while assembling
.Ic OPT
can be used to change some of the options during assembling the
source instead of defining them on the commandline.
.Pp
.Ic OPT
takes a comma-seperated list of options as its argument:
.Pp
.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:
.Sy b , e
and
.Sy g .
.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 stacks number of entries is limited only by the amount of memory in your
machine.
.Sh SEE ALSO
.Xr rgbasm 1 ,
.Xr rgblink 1 ,
.Xr rgblink 5 ,
.Xr rgbds 5 ,
.Xr rgbds 7 ,
.Xr gbz80 7
.Sh HISTORY
.Nm rgbds
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/rednex/rgbds .