New code for the shift functions

git-svn-id: svn://svn.cc65.org/cc65/trunk@3143 b7a2c559-68d2-44c3-8de9-860c34a00d81

libsrc/runtime/asr.s

@@ -0,0 +1,60 @@
+;
+; Ullrich von Bassewitz, 2004-06-30
+;
+; CC65 runtime: right shift support for ints
+;
+; 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!
+;
+
+
+      	.export		tosasrax
+      	.import		popax
+      	.importzp	tmp1
+
+tosasrax:
+        and     #$0F            ; Bring the shift count into a valid range
+        sta     tmp1            ; Save it
+
+        jsr     popax           ; Get the left hand operand
+
+        ldy     tmp1            ; Get shift count
+        beq     L9              ; Bail out if shift count zero
+
+        cpy     #8              ; Shift count 8 or greater?
+        bcc     L1              ; Jump if not
+
+; Shift count is greater 8. The carry is set when we enter here.
+
+        tya
+        sbc     #8
+        tay                     ; Adjust shift count
+        txa
+        ldx     #$00            ; Shift by 8 bits
+        cmp     #$00            ; Test sign bit
+        bpl     L1
+        dex                     ; Make X the correct sign extended value
+
+; Save the high byte so we can shift it
+
+L1:     stx     tmp1            ; Save high byte
+        jmp     L3
+
+; Do the actual shift
+
+L2:    	cpx     #$80            ; Copy bit 15 into the carry
+        ror     tmp1
+        ror     a
+L3:     dey
+       	bpl     L2
+
+; Done with shift
+
+        ldx    	tmp1
+L9:     rts
+
+
+