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https://github.com/cc65/cc65.git
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128 lines
2.0 KiB
ArmAsm
128 lines
2.0 KiB
ArmAsm
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; Ullrich von Bassewitz, 05.08.1998
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;
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; CC65 runtime: right shift support for ints
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;
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; --------------------------------------------------------------------
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; signed shift
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.export tosasra0, tosasrax
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.import popsreg, return0
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.importzp sreg
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tosasra0:
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ldx #0
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tosasrax:
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jsr popsreg ; get TOS into sreg
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cpx #0
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bne TooLarge
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cmp #16
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bcs TooLarge
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cmp #8 ; Shift count greater 8?
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beq L4 ; Jump if exactly 8
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bcc L2 ; Jump if no
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; Shift count is greater 8. Do the first 8 bits the fast way
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ldy #0
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ldx sreg+1
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stx sreg
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bpl L1
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dey ; Create correct sign bits
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L1: sty sreg+1
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sec
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sbc #8
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; Shift count less than 8 if we come here
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L2: tay ; Shift count --> Y
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beq Zero ; Done if shift count zero
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lda sreg ; get low byte for faster shift
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ldx sreg+1 ; get high byte
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; Do the actual shift
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L3: cpx #$80 ; get bit 7 into carry
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ror sreg+1
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ror a
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dey
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bne L3
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; Done with shift
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ldx sreg+1
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rts
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; Shift count == 8
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L4: lda sreg+1 ; X is zero
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bpl *+3
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dex ; X == 0xFF
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rts
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; Shift count was zero
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Zero: lda sreg
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ldx sreg+1
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rts
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; Shift count too large, result is zero
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TooLarge:
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jmp return0
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; --------------------------------------------------------------------
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; unsigned shift
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.export tosshra0, tosshrax
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tosshra0:
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ldx #0
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tosshrax:
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jsr popsreg ; get TOS into sreg
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cpx #0
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bne TooLarge
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cmp #16
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bcs TooLarge
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cmp #8 ; Shift count greater 8?
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beq L8 ; Jump if exactly 8
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bcc L6 ; Jump if no
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; Shift count is greater 8. Do the first 8 bits the fast way
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sbc #8 ; Carry already set
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ldy sreg+1
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sty sreg
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stx sreg+1 ; High byte = 0
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; Shift count less than 8 if we come here
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L6: tay ; Shift count --> Y
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beq Zero ; Done if shift count zero
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lda sreg ; get low byte for faster shift
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; Do the actual shift
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L7: lsr sreg+1
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ror a
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dey
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bne L7
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; Done with shift
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ldx sreg+1
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rts
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; Shift count == 8
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L8: lda sreg+1 ; X is zero
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rts
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