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ef3cbbe340
git-svn-id: https://svn.code.sf.net/p/acme-crossass/code-0/trunk@134 4df02467-bbd4-4a76-a152-e7ce94205b78
89 lines
3.0 KiB
Plaintext
89 lines
3.0 KiB
Plaintext
;ACME 0.94.5
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!ifdef lib_cbm_mflpt_a !eof
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lib_cbm_mflpt_a = 1
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; CAUTION! The Commodore BASIC interpreter uses two different formats for
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; handling floating-point values, so do not confuse them:
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; The "float registers" fac1 and fac2 (actually structures in zero page) use a
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; six-byte format commonly known as "flpt" (floating point).
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; When storing values in variables (or reading values from ROM), a compressed
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; five-byte format is used, commonly known as "mflpt" (memory floating point).
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; This file contains a macro for writing floating point numbers in the five-byte
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; "mflpt" format, where the sign bit is packed into the mantissa.
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; Several interpreter functions use this format (see <cbm/c64/float.a>).
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; Use the macro like this:
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; +mflpt 3.1415926 ; each use will take up five bytes of memory
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; now for the technical stuff (stop reading right now if you value your sanity)
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; five-byte layout in memory:
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; eeeeeeee smmmmmmm mmmmmmmm mmmmmmmm mmmmmmmm ; eight bits exponent, 32 bits mantissa with sign bit overlay
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; exponent byte:
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; exponent has a bias of 128 (128 means the decimal point is right before the mantissa's leading digit)
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; if exponent is zero, number value is considered to be zero, regardless of mantissa
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; exponents 1..128 are for values < 1
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; exponents 129..255 are for values >= 1
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; mantissa:
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; mantissa is stored big-endian(!)
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; the mantissa's mandatory leading '1' is replaced by the sign bit
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; so logically, this is equivalent to:
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; + .1mmmmmmm mmmmmmmm mmmmmmmm mmmmmmmm * 2^(eeeeeeee - 128) if sign bit is 0
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; - .1mmmmmmm mmmmmmmm mmmmmmmm mmmmmmmm * 2^(eeeeeeee - 128) if sign bit is 1
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; this is ugly, but it gets the job done
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; (if it's stupid, but it works, then it's not stupid)
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!macro mflpt @value {
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!set @float = float(@value)
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!ifndef @float {
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!by <@float, $ff, $ff, $ff, $ff ; five place holder bytes
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; (first one depends on @float just to make sure more passes are done until value is defined)
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} else {
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; value is defined, so split up into sign and non-negative value
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!if @float < 0 {
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!set @sign = $80
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!set @float = -@float
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} else {
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!set @sign = $00
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}
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!if @float = 0 {
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!by 0, 0, 0, 0, 0 ; five zeroes (zero is represented by all bits zero)
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} else {
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; split up into exponent and mantissa
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!set @exponent = 128 + 32 ; 128 is cbm's bias, 32 is this algo's bias
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; if mantissa is too large, shift right and adjust exponent
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!do while @float >= (2.0 ^ 32.0) {
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!set @float = @float >> 1
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!set @exponent = @exponent + 1
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}
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; if mantissa is too small, shift left and adjust exponent
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!do while @float < (2.0 ^ 31.0) {
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!set @float = @float << 1
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!set @exponent = @exponent - 1
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}
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!if @exponent < 1 {
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!warn "MFLPT underflow, using zero instead"
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!set @float = 0
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!set @exponent = 0
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!set @sign = 0
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}
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!if @exponent > 255 {
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!error "MFLPT overflow"
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}
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!by @exponent
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!by (127 & int(@float >> 24)) | @sign
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!by 255 & int(@float >> 16)
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!by 255 & int(@float >> 8)
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!by 255 & int(@float)
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}
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}
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}
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