New code for the shift functions

git-svn-id: svn://svn.cc65.org/cc65/trunk@3143 b7a2c559-68d2-44c3-8de9-860c34a00d81
2026-04-25 06:17:58 +00:00 · 2004-07-05 22:24:06 +00:00
parent 07419b62f8
commit c122f18605
7 changed files with 240 additions and 406 deletions
@@ -1,127 +1,55 @@
 ;
-; Ullrich von Bassewitz, 05.08.1998
+; Ullrich von Bassewitz, 2004-06-30
 ;
-; CC65 runtime: right shift support for ints
+; CC65 runtime: right shift support for unsigneds
+;
+; Note: The standard declares a shift count that is negative or >= the
+; bitcount of the shifted type for undefined behaviour.
+;
+; Note^2: The compiler knowns about the register/zero page usage of this
+; function, so you need to change the compiler source if you change it!
 ;


-; --------------------------------------------------------------------
-; signed shift
+	.export		tosshrax
+	.import		popax
+	.importzp	tmp1

-  	.export		tosasra0, tosasrax
-	.import		popsreg, return0
-	.importzp	sreg
-
-tosasra0:
-  	ldx	#0
-tosasrax:
-  	jsr	popsreg		; get TOS into sreg
-  	cpx	#0
-  	bne	TooLarge
-  	cmp	#16
-  	bcs	TooLarge
-
-       	cmp 	#8		; Shift count greater 8?
-	beq	L4		; Jump if exactly 8
-       	bcc    	L2		; Jump if no
-
-; Shift count is greater 8. Do the first 8 bits the fast way
-
-   	ldy    	#0
-   	ldx	sreg+1
-   	stx	sreg
-   	bpl	L1
-   	dey	  		; Create correct sign bits
-L1:	sty	sreg+1
-   	sec
-   	sbc	#8
-
-; Shift count less than 8 if we come here
-
-L2:	tay			; Shift count --> Y
-	beq    	Zero		; Done if shift count zero
-
-  	lda	sreg		; get low byte for faster shift
-  	ldx	sreg+1		; get high byte
-
-; Do the actual shift
-
-L3:	cpx	#$80		; get bit 7 into carry
-  	ror	sreg+1
-  	ror	a
-	dey
-       	bne	L3
-
-; Done with shift
-
-  	ldx	sreg+1
-  	rts
-
-; Shift count == 8
-
-L4:	lda	sreg+1		; X is zero
-	bpl	*+3
-	dex			; X == 0xFF
-	rts
-
-; Shift count was zero
-
-Zero:	lda	sreg
-	ldx	sreg+1
-	rts
-
-; Shift count too large, result is zero
-
-TooLarge:
-	jmp	return0
-
-
-; --------------------------------------------------------------------
-; unsigned shift
-
-  	.export		tosshra0, tosshrax
-
-tosshra0:
-      	ldx	#0
 tosshrax:
-  	jsr	popsreg		; get TOS into sreg
-  	cpx	#0
-  	bne	TooLarge
-  	cmp	#16
-  	bcs	TooLarge
+        and     #$0F            ; Bring the shift count into a valid range
+        sta     tmp1            ; Save it

-       	cmp 	#8		; Shift count greater 8?
-	beq	L8		; Jump if exactly 8
-       	bcc    	L6		; Jump if no
+        jsr     popax           ; Get the left hand operand

-; Shift count is greater 8. Do the first 8 bits the fast way
+        ldy     tmp1            ; Get shift count
+        beq     L9              ; Bail out if shift count zero

-   	sbc	#8		; Carry already set
-   	ldy	sreg+1
-   	sty	sreg
-       	stx	sreg+1		; High byte = 0
+        cpy     #8              ; Shift count 8 or greater?
+        bcc     L3              ; Jump if not

-; Shift count less than 8 if we come here
+; Shift count is greater 7. The carry is set when we enter here.

-L6:	tay	  		; Shift count --> Y
-  	beq    	Zero		; Done if shift count zero
+        tya
+        sbc     #8
+        tay                     ; Adjust shift count
+        txa
+        ldx     #$00            ; Shift by 8 bits
+        beq     L2              ; Branch always
+L1:     lsr     a
+L2:     dey
+        bpl     L1
+        rts

-  	lda	sreg		; get low byte for faster shift
+; Shift count is less than 8. Do the actual shift.

-; Do the actual shift
-
-L7:    	lsr	sreg+1
-  	ror	a
-  	dey
-       	bne	L7
+L3:     stx     tmp1            ; Save high byte of lhs
+L4:     lsr     tmp1
+        ror     a
+        dey
+       	bne     L4

 ; Done with shift

-  	ldx	sreg+1
-  	rts
-
-; Shift count == 8
-
-L8:	lda	sreg+1		; X is zero
-	rts
+        ldx	tmp1
+L9:     rts