86 lines
1.6 KiB
ArmAsm
86 lines
1.6 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 - add FAC2 to FAC1
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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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// Note:
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// - load FAC2 after FAC1, or mimic the Kernals sign comparison (XXX do we need it?)
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//
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// See also:
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// - [CM64] Computes Mapping the Commodore 64 - page 112
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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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add_FAC2_FAC1:
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// Check if FAC2 is 0; if so leave FAC1 without changes
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lda FAC2_exponent
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beq add_FAC2_FAC1_done
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// If FAC1 is 0, if so just copy FAC2 to FAC1
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lda FAC1_exponent
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beq_16 mov_FAC2_FAC1
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// Make sure the exponents are aligned
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jsr add_align_exponents
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// Check whether signs are the same
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lda FAC1_sign
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eor FAC2_sign
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bmi_16 add_FAC2_FAC1_sub // branch if signs are opposite
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// Perform the addition
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clc
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lda FAC2_mantissa+3
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adc FAC1_mantissa+3
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sta FAC1_mantissa+3
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lda FAC2_mantissa+2
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adc FAC1_mantissa+2
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sta FAC1_mantissa+2
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lda FAC2_mantissa+1
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adc FAC1_mantissa+1
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sta FAC1_mantissa+1
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lda FAC2_mantissa+0
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adc FAC1_mantissa+0
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sta FAC1_mantissa+0
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bcc add_FAC2_FAC1_normalize // end if no need to adapt the exponent
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inc FAC1_exponent
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bne add_FAC2_FAC1_normalize // end if no overflow
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lda #$FF
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sta FAC1_exponent
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sta FAC1_mantissa+3
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sta FAC1_mantissa+2
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sta FAC1_mantissa+1
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sta FAC1_mantissa+0
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// FALLTROUGH
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add_FAC2_FAC1_done:
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rts
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add_FAC2_FAC1_normalize:
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jmp normal_FAC1
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