62 lines
1.2 KiB
ArmAsm
62 lines
1.2 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 - rounds FAC1 before copying
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//
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// See also:
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// - [CM64] Computes Mapping the Commodore 64 - page 115
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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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round_FAC1:
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// If number is 0, do not do anything
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lda FAC1_exponent
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beq round_FAC1_done
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lda FACOV
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bpl round_FAC1_zero_FACOV // branch if it is enough to clear FACOV
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// Increment the 4 bytes of matissa
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// Curse you, who invented this format! Had the mantissa been little endian,
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// we could have used INW on 65C02 and above, the code would have been smaller!
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inc FAC1_mantissa+3
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bne round_FAC1_zero_FACOV
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inc FAC1_mantissa+2
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bne round_FAC1_zero_FACOV
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inc FAC1_mantissa+1
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bne round_FAC1_zero_FACOV
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inc FAC1_mantissa+0
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bne round_FAC1_zero_FACOV
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// If we are here, it means mantissa is 0; put it to minimum allowed and increment exponent
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lda #$80
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sta FAC1_mantissa+0
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inc FAC1_exponent
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bne round_FAC1_zero_FACOV
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// If we are here, it means the exponent wrapped around to $00; put it back to $FF
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dec FAC1_exponent
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// FALLTROUGH
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round_FAC1_zero_FACOV:
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lda #$00
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sta FACOV
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// FALLTROUGH
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round_FAC1_done:
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rts
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