109 lines
2.1 KiB
ArmAsm
109 lines
2.1 KiB
ArmAsm
// #LAYOUT# STD * #TAKE
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// #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
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// #LAYOUT# * BASIC_0 #TAKE
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// #LAYOUT# * * #IGNORE
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//
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// Math package - divide FAC2 (dividend) by FAC1 (divisor), ignores sign
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//
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// Input:
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// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
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//
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// Output:
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// - leaves the quotient FAC1 and the remainder in FAC2
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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 provide implementation
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div_FAC2_FAC1:
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STUB_IMPLEMENTATION()
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// XXX move this to a separate file
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// Unsigned integer 32-bit mantissa division, based on code by Verz
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// divident: FAC2_mantissa, divisor: FAC1_mantissa, result: FAC2_mantissa, remainder: RESHO
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// temporary storage: INDEX+1, INDEX+2, INDEX+3
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//
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// https://codebase64.org/doku.php?id=base:24bit_division_24-bit_result
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// XXX test this, optimize (for temporary storage use .Y, .Z and RESHO+4)
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div_mantissas:
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// Preset remainder to 0
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// XXX should be common with multiplication
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lda #$00
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sta RESHO+0
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sta RESHO+1
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sta RESHO+2
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sta RESHO+3
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ldx #$32 // repeat for each bit
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// FALLTROUGH
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div_mantissas_loop:
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// Multiply divident by 2, msb -> Carry
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asl FAC2_mantissa+3
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rol FAC2_mantissa+2
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rol FAC2_mantissa+1
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rol FAC2_mantissa+0
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// Multiply remainder by 2, lsb <- Carry
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rol RESHO+3
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rol RESHO+2
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rol RESHO+1
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rol RESHO+0
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// Subtract divisor to see if it fits in
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lda RESHO+3
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sec
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sbc FAC1_mantissa+2
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sta INDEX+3
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lda RESHO+2
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sbc FAC1_mantissa+2
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sta INDEX+2
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lda RESHO+1
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sbc FAC1_mantissa+1
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sta INDEX+1
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lda RESHO+0
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sbc FAC1_mantissa+0
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bcc div_mantissas_skip // if Carry is not set then divisor did not fit in yet
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sta RESHO+0 // else save substraction result as new remainder,
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lda INDEX+1
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sta RESHO+1
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lda INDEX+2
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sta RESHO+2
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lda INDEX+3
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sta RESHO+3
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inc FAC2_mantissa+3 // and INCrement result cause divisor fit in 1 times
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// FALLTROUGH
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div_mantissas_skip:
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dex
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bne div_mantissas_loop
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rts
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