102 lines
2.0 KiB
ArmAsm
102 lines
2.0 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 - normalize FAC1
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//
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// See also:
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// - [CM64] Computes Mapping the Commodore 64 - page 113
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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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normal_FAC1:
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lda FAC1_exponent
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beq normal_FAC1_end // branch if our float is 0
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// FALLTROUGH
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normal_FAC1_check_mantissa_byte:
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lda FAC1_mantissa+0
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beq normal_FAC1_by_byte // branch if we can move the whole byte
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// FALLTROUGH
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normal_FAC1_check_mantissa_bit:
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lda FAC1_mantissa+0 // XXX optimize for 65CE02?
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and #$80
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bne normal_FAC1_end // branch if already normalized
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// FALLTROUGH
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normal_FAC1_by_bit:
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// Decrement the exponent
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dec FAC1_exponent
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beq normal_FAC1_end // exponent reached 0, nothing more to do
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// Multiply the mantissa by 2
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asl FACOV
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rol FAC1_mantissa+3
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rol FAC1_mantissa+2
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rol FAC1_mantissa+1
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rol FAC1_mantissa+0
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bcc normal_FAC1_check_mantissa_bit // branch if next iteration needed
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rts
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normal_FAC1_by_byte:
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// First decrement the exponent by 8, but not below 0
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lda FAC1_exponent
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sec
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sbc #$08
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bcs !+
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lda #$00
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!:
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sta FAC1_exponent
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beq normal_FAC1_end // branch if our float reached 0
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// Now multiply the mantissa by 8 by moving the whole bytes
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lda FAC1_mantissa+1
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sta FAC1_mantissa+0
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lda FAC1_mantissa+2
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sta FAC1_mantissa+1
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lda FAC1_mantissa+3
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sta FAC1_mantissa+2
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lda FACOV
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sta FAC1_mantissa+3
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lda #$00
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sta FACOV
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// Now check if mantissa is not 0
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ora FAC1_mantissa+0
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ora FAC1_mantissa+1
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ora FAC1_mantissa+2
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ora FAC1_mantissa+3
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bne normal_FAC1_check_mantissa_byte // not 0 - branch to next iteration
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// Our mantissa reached 0 - mark exponent as 0 and finish
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sta FAC1_exponent
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// FALLTROUGH
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normal_FAC1_end:
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rts
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