81 lines
1.7 KiB
ArmAsm
81 lines
1.7 KiB
ArmAsm
// #LAYOUT# STD * #TAKE
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// #LAYOUT# * BASIC_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// Math package - fetch FAC2 from RAM location
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//
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// Input:
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// - .A - address low byte
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// - .Y - address high byte
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//
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// Output:
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// - .A - FAC1 exponent (yes, really - Codebase64 is right)
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// - .Y - 0 (experimentation with original ROM)
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//
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// Preserves:
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// - .X - (experimentation with original ROM)
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//
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// See also:
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// - [CM64] Computes Mapping the Commodore 64 - page 114
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// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
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// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
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//
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// XXX test this
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CONUPK:
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// Checked in the available documentation:
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// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
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// - https://www.lemon64.com/forum/viewtopic.php?t=67576&sid=d41e77cdddd28d6f9c9d52ce5b4e8dc3
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// that $22/$23 (INDEX+0 and INDEX+1) are at least sometimes used for pointer storage.
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// Confirmed by experimenting with original ROM that this is the case here too.
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sty INDEX+1
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sta INDEX+0
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// Now copy the data from RAM to FAC2, for the format description see:
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// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic#Representation_in_the_C-64
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// Copy the mantissa
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ldy #$04
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lda (INDEX), y
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sta FAC2_mantissa+3
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dey
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lda (INDEX), y
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sta FAC2_mantissa+2
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dey
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lda (INDEX), y
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sta FAC2_mantissa+1
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dey
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lda (INDEX), y
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ora #$80 // clear the sign bit
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sta FAC2_mantissa+0
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// Copy the sign
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lda (INDEX), y
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bpl !+ // assumption: non-negative numbers are more frequent
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lda #$FF
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skip_2_bytes_trash_nvz
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!:
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lda $00
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sta FAC2_sign
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// Copy the exponent
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dey
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lda (INDEX), y
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sta FAC2_exponent
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// Return the FAC1 exponent
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lda FAC1_exponent
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rts
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