mirror of
https://github.com/uffejakobsen/acme.git
synced 2024-11-29 19:50:12 +00:00
ef3cbbe340
git-svn-id: https://svn.code.sf.net/p/acme-crossass/code-0/trunk@134 4df02467-bbd4-4a76-a152-e7ce94205b78
95 lines
3.2 KiB
Plaintext
95 lines
3.2 KiB
Plaintext
;ACME 0.96.4
|
|
|
|
!ifdef lib_cbm_flpt_a !eof
|
|
lib_cbm_flpt_a = 1
|
|
|
|
; CAUTION! The Commodore BASIC interpreter uses two different formats for
|
|
; handling floating-point values, so do not confuse them:
|
|
; The "float registers" fac1 and fac2 (actually structures in zero page) use a
|
|
; six-byte format commonly known as "flpt" (floating point).
|
|
; When storing values in variables (or reading values from ROM), a compressed
|
|
; five-byte format is used, commonly known as "mflpt" (memory floating point).
|
|
|
|
; This file contains a macro for writing floating point numbers in the six-byte
|
|
; "flpt" format, where the sign bit occupies the sixth byte.
|
|
; There are no interpreter functions to use this format, so you will have to
|
|
; write your own functions for "copy-mem-to-fac1", "copy-fac2-to-mem" etc.
|
|
|
|
; Use the macro like this:
|
|
; +flpt 3.1415926 ; each use will take up six bytes of memory
|
|
|
|
|
|
; now for the technical stuff (stop reading right now if you value your sanity)
|
|
|
|
; six-byte layout in memory:
|
|
; eeeeeeee 1mmmmmmm mmmmmmmm mmmmmmmm mmmmmmmm sxxxxxxx ; eight bits exponent, 32 bits mantissa with leading '1', sign byte
|
|
|
|
; exponent byte:
|
|
; exponent has a bias of 128 (128 means the decimal point is right before the mantissa's leading digit)
|
|
; if exponent is zero, number value is considered to be zero, regardless of mantissa
|
|
; exponents 1..128 are for values < 1
|
|
; exponents 129..255 are for values >= 1
|
|
|
|
; mantissa:
|
|
; mantissa is stored big-endian(!)
|
|
; the mantissa's leading digit is always '1' (unless the whole value represents zero)
|
|
|
|
; sign byte:
|
|
; most significant bit is sign: 0 means positive number, 1 means negative number
|
|
; the seven lower bits are unused
|
|
|
|
; so logically, this is equivalent to:
|
|
; + .1mmmmmmm mmmmmmmm mmmmmmmm mmmmmmmm * 2^(eeeeeeee - 128) if sign bit is 0
|
|
; - .1mmmmmmm mmmmmmmm mmmmmmmm mmmmmmmm * 2^(eeeeeeee - 128) if sign bit is 1
|
|
|
|
|
|
|
|
; this is ugly, but it gets the job done
|
|
; (if it's stupid, but it works, then it's not stupid)
|
|
!macro flpt @value {
|
|
!set @float = float(@value)
|
|
!ifndef @float {
|
|
!by <@float, $ff, $ff, $ff, $ff, $ff ; six place holder bytes
|
|
; (first one depends on @float just to make sure more passes are done until value is defined)
|
|
} else {
|
|
; value is defined, so split up into sign and non-negative value
|
|
!if @float < 0 {
|
|
!set @sign = $80
|
|
!set @float = -@float
|
|
} else {
|
|
!set @sign = $00
|
|
}
|
|
!if @float = 0 {
|
|
!by 0, 0, 0, 0, 0, 0 ; six zeroes (zero is represented by all bits zero)
|
|
} else {
|
|
; split up into exponent and mantissa
|
|
!set @exponent = 128 + 32 ; 128 is cbm's bias, 32 is this algo's bias
|
|
; if mantissa is too large, shift right and adjust exponent
|
|
!do while @float >= (2.0 ^ 32.0) {
|
|
!set @float = @float >> 1
|
|
!set @exponent = @exponent + 1
|
|
}
|
|
; if mantissa is too small, shift left and adjust exponent
|
|
!do while @float < (2.0 ^ 31.0) {
|
|
!set @float = @float << 1
|
|
!set @exponent = @exponent - 1
|
|
}
|
|
!if @exponent < 1 {
|
|
!warn "FLPT underflow, using zero instead"
|
|
!set @float = 0
|
|
!set @exponent = 0
|
|
!set @sign = 0
|
|
}
|
|
!if @exponent > 255 {
|
|
!error "FLPT overflow"
|
|
}
|
|
!by @exponent
|
|
!by 255 & int(@float >> 24)
|
|
!by 255 & int(@float >> 16)
|
|
!by 255 & int(@float >> 8)
|
|
!by 255 & int(@float)
|
|
!by @sign
|
|
}
|
|
}
|
|
}
|