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c3e12f91db
git-svn-id: https://svn.code.sf.net/p/acme-crossass/code-0/trunk@61 4df02467-bbd4-4a76-a152-e7ce94205b78
94 lines
3.1 KiB
Plaintext
94 lines
3.1 KiB
Plaintext
;ACME 0.94.5
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!ifdef lib_cbm_flpt_a !eof
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lib_cbm_flpt_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 six-byte
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; "flpt" format, where the sign bit occupies the sixth byte.
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; There are no interpreter functions to use this format, so you will have to
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; write your own functions for "copy-mem-to-fac1", "copy-fac2-to-mem" etc.
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; Use the macro like this:
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; +flpt 3.1415926 ; each use will take up six 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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; six-byte layout in memory:
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; eeeeeeee 1mmmmmmm mmmmmmmm mmmmmmmm mmmmmmmm sxxxxxxx ; eight bits exponent, 32 bits mantissa with leading '1', sign byte
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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 leading digit is always '1' (unless the whole value represents zero)
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; sign byte:
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; most significant bit is sign: 0 means positive number, 1 means negative number
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; the seven lower bits are unused
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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 flpt .value {
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!set .float = float(.value) ; make sure to do passes until value is defined
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!ifndef .float {
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!by $ff, $ff, $ff, $ff, $ff, $ff ; six place holder bytes
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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, 0 ; six 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 "FLPT 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 "FLPT overflow"
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}
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!by .exponent
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!by 255 & int(.float >> 24)
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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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!by .sign
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}
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}
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}
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