168 lines
2.8 KiB
ArmAsm
168 lines
2.8 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 - align FAC1 and FAC2 exponents for addition
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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://www.c64-wiki.com/wiki/BASIC-ROM
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add_align_exponents:
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// XXX assume here none of the exponents is 0, check if this is true for orignal ROMs
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lda FAC1_exponent
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cmp FAC2_exponent
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beq add_align_exponents_done // branch if exponents the same
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bcs add_align_exponent_FAC2
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// FALLTROUGH
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add_align_exponent_FAC1:
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// First check the exponent difference
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lda FAC2_exponent
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sec
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sbc FAC1_exponent
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cmp #$28
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bcs add_align_exponent_FAC1_zero // branch if difference is very high
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cmp #$08
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bcc add_align_exponent_FAC1_bit
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// FALLTROUGH
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add_align_exponent_FAC1_byte:
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// Add 8 to exponent, divide mantissa by 256
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lda FAC1_exponent
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clc
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adc #$08
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sta FAC1_exponent
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lda FAC1_mantissa+3
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sta FACOV
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lda FAC1_mantissa+2
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sta FAC1_mantissa+3
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lda FAC1_mantissa+1
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sta FAC1_mantissa+2
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lda FAC1_mantissa+0
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sta FAC1_mantissa+1
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lda #$00
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sta FAC1_mantissa+0
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beq add_align_exponent_FAC1 // branch always - next iteration
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add_align_exponent_FAC1_bit:
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// Increment exponent, divide mantissa by 2
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inc FAC1_exponent
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clc
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ror FAC1_mantissa+0
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ror FAC1_mantissa+1
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ror FAC1_mantissa+2
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ror FAC1_mantissa+3
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ror FACOV
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lda FAC1_exponent
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cmp FAC2_exponent
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bne add_align_exponent_FAC1_bit // branch if next iteration needed
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rts
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add_align_exponent_FAC1_zero:
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lda #$00
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sta FAC1_mantissa+0
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sta FAC1_mantissa+1
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sta FAC1_mantissa+2
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sta FAC1_mantissa+3
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sta FACOV
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lda FAC2_exponent
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sta FAC1_exponent
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// FALLTROUGH
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add_align_exponents_done:
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rts
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add_align_exponent_FAC2:
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// First check the exponent difference, Carry already set here
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lda FAC1_exponent
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sbc FAC2_exponent
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cmp #$20
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bcs add_align_exponent_FAC2_zero // branch if difference is very high
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cmp #$08
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bcc add_align_exponent_FAC2_bit
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// FALLTROUGH
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add_align_exponent_FAC2_byte:
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// Add 8 to exponent, divide mantissa by 256
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lda FAC2_exponent
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clc
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adc #$08
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sta FAC2_exponent
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lda FAC2_mantissa+2
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sta FAC2_mantissa+3
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lda FAC2_mantissa+1
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sta FAC2_mantissa+2
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lda FAC2_mantissa+0
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sta FAC2_mantissa+1
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lda #$00
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sta FAC2_mantissa+0
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beq add_align_exponent_FAC2 // branch always - next iteration
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add_align_exponent_FAC2_bit:
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// Increment exponent, divide mantissa by 2
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inc FAC2_exponent
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clc
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ror FAC2_mantissa+0
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ror FAC2_mantissa+1
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ror FAC2_mantissa+2
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ror FAC2_mantissa+3
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lda FAC1_exponent
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cmp FAC2_exponent
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bne add_align_exponent_FAC2_bit // branch if next iteration needed
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rts
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add_align_exponent_FAC2_zero:
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lda #$00
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sta FAC2_mantissa+0
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sta FAC2_mantissa+1
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sta FAC2_mantissa+2
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sta FAC2_mantissa+3
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lda FAC1_exponent
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sta FAC2_exponent
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rts
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