Improve fixed-point documentation (#1455)

* Clarify the operator relationship of ordinary and fixed-point numbers
* Attempt to clarify description of fixed-point numbers
* Note that RGBASM does not check fixed-point precisions
* Simplify sine table example a bit
* Remove misleading equations describing `DIV`, `MUL`, and `FMOD`
* Various minor style and formatting fixups

man/rgbasm.5

@@ -369,23 +369,28 @@ equals $roman clz ( n )$.
 delim off
 .EN
 .Ss Fixed-point expressions
-Fixed-point numbers are basically normal (32-bit) integers, which count fractions instead of whole numbers.
-They offer better precision than integers but limit the range of values.
-By default, the upper 16 bits are used for the integer part and the lower 16 bits are used for the fraction (65536ths).
-The default number of fractional bits can be changed with the
+Fixed-point numbers are technically just integers, but conceptually they have a decimal point at a fixed location (hence the name).
+This gives them increased precision, at the cost of a smaller range, while remaining far cheaper to manipulate than floating-point numbers (which
+.Nm
+does not support).
+.Pp
+The default precision of all fixed-point numbers is 16 bits, meaning the lower 16 bits are used for the fractional part; so they count in 65536ths of 1.0.
+This precision can be changed with the
 .Fl Q
-command-line option.
-You can also specify a precise fixed-point value by appending a
+command-line option, and/or by
+.Ic OPT Q
+.Pq see Sx Changing options while assembling .
+An individual fixed-point literal can specify its own precision, overriding the current default, by appending a
 .Dq q
-to it followed by the number of fractional bits, such as
-.Ql 12.34q8 .
+followed by the number of fractional bits: for example,
+.Ql 1234.5q8
+is equal to $0004d2_80
+.EQ
+delim $$
+.EN
+($= 1234.5 * 2 sup 8$).
 .Pp
 Since fixed-point values are still just integers, you can use them in normal integer expressions.
-Some integer operators like
-.Sq +
-and
-.Sq -
-don't care whether the operands are integers or fixed-point.
 You can easily truncate a fixed-point number into an integer by shifting it right by the number of fractional bits.
 It follows that you can convert an integer to a fixed-point number by shifting it left that same amount.
 .Pp
@@ -393,19 +398,16 @@ Note that the current number of fractional bits can be computed as
 .Ic TZCOUNT Ns Pq 1.0 .
 .Pp
 The following functions are designed to operate with fixed-point numbers:
-.EQ
-delim $$
-.EN
 .Bl -column -offset indent "ATAN2(y, x)"
 .It Sy Name Ta Sy Operation
-.It Fn DIV x y Ta Fixed-point division $( x \[di] y ) \[mu] ( 2 ^ precision )$
-.It Fn MUL x y Ta Fixed-point multiplication $( x \[mu] y ) \[di] ( 2 ^ precision )$
-.It Fn FMOD x y Ta Fixed-point modulo $( x % y ) \[di] ( 2 ^ precision )$
-.It Fn POW x y Ta $x$ to the $y$ power
+.It Fn DIV x y Ta Fixed-point division
+.It Fn MUL x y Ta Fixed-point multiplication
+.It Fn FMOD x y Ta Fixed-point modulo
+.It Fn POW x y Ta $x sup y$
 .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 CEIL x Ta Round $x$ up to the nearest integer
+.It Fn FLOOR x Ta Round $x$ down to the nearest integer
 .It Fn SIN x Ta Sine of $x$
 .It Fn COS x Ta Cosine of $x$
 .It Fn TAN x Ta Tangent of $x$
@@ -418,7 +420,25 @@ delim $$
 delim off
 .EN
 .Pp
-All of these fixed-point functions can take an optional final argument, which is the precision to use.
+There are no functions for fixed-point addition and subtraction, because the
+.Sq +
+and
+.Sq -
+operators can add and subtract pairs of fixed-point operands.
+.Bd -ragged -offset indent
+Note that some operators or functions are meaningful when combining integers and fixed-point values.
+For example,
+.Ql 2.0 * 3
+is equivalent to
+.Ql MUL(2.0, 3.0) ,
+and
+.Ql 6.0 / 2
+is equivalent to
+.Ql DIV(6.0, 2.0) .
+Be careful and think about what the operations mean when doing this sort of thing.
+.Ed
+.Pp
+All of these fixed-point functions can take an optional final argument, which is the precision to use for that one operation.
 For example,
 .Ql MUL(6.0q8, 7.0q8, 8)
 will evaluate to
@@ -426,6 +446,12 @@ will evaluate to
 no matter what value is set as the current
 .Cm Q
 option.
+.Nm
+.Em does not check precisions for consistency ,
+so nonsensical input like
+.Ql MUL(4.2q8, 6.9q12, 16)
+will produce a nonsensical (but technically correct) result:
+.Dq garbage in, garbage out .
 .Pp
 The
 .Ic FMOD
@@ -439,21 +465,26 @@ this is the opposite of how the integer modulo operator
 .Sq %
 works!
 .Pp
-The trigonometry functions (
-.Ic SIN ,
-.Ic COS ,
-.Ic TAN ,
-etc) are defined in terms of a circle divided into 1.0 "turns" (equal to 2pi radians or 360 degrees).
+The trigonometry functions
+.Pq Ic SIN , Ic COS , Ic TAN , No etc
+are defined in terms of a circle divided into 1.0
+.Dq turns
+.EQ
+delim $$
+.EN
+(equal to $2 pi$ radians, or 360 degrees).
+.EQ
+delim off
+.EN
 .Pp
 These functions are useful for automatic generation of various tables.
 For example:
 .Bd -literal -offset indent
-; Generate a table of sine values from sin(0.0) to sin(1.0), with
-; amplitude scaled from [-1.0, 1.0] to [0.0, 128.0]
-DEF turns = 0.0
-REPT 256
-    db MUL(64.0, SIN(turns) + 1.0) >> 16
-    DEF turns += 1.0 / 256
+; Generate a table of 128 sine values
+; from sin(0.0) to sin(0.5) excluded,
+; with amplitude scaled from [-1.0, 1.0] to [0.0, 128.0].
+FOR angle, 0.0, 0.5, 0.5 / 128
+    db MUL(SIN(angle) + 1.0, 128.0 / 2) >> 16
 ENDR
 .Ed
 .Ss String expressions