Implemented a PEI slammer that can break for interrupt handling

src/blitter/PEISlammer.s

@@ -0,0 +1,116 @@
+; Implementation of a PEI Slammer that updates a rectangular screen area.  The only tweak that
+; this implementation does is that it does break up the slam into chunks of scan lines to allow
+; time for interrupts to be serviced in a timely manner.
+;
+; A = base address of top-left edge of the screen
+; Y = number of scanlines to blit
+; X = width of the screen in bytes
+PEISlam
+               stx   :screen_width  ; save the width
+
+               phd                  ; save the current direct page and assign the base
+               tcd                  ; creen address to the direct page register
+
+               tsc
+               sta   :stk_save      ; save the stack pointer to restore later
+
+               lda   #:pei_end      ; patch the PEI entry address
+               sec
+               sbc   :screen_width
+               sta   :inner+1
+
+               clc                  ; clear before the loop -- nothing in the loop affect the carry bit
+               bra   :outer         ; hop into the entry point.  The loop control logic is next because 
+                                    ; the size of the PEI instruction is too large to use short branches
+                                    ; in the code after pei_end
+
+:control
+               tdc                  ; Move to the next line
+               adc   #160
+               tcd
+               adc   :screen_width
+               tcs
+
+               dey                  ; decrement the total counter, if zero then we're done
+               beq   :exit
+
+               dex                  ; decrement the inner counter
+               bne   :inner         ; if not zero, no break; go to the next line
+
+               _R0W0                ; restore the execution environment and
+               lda   :stk_save      ; give a few cycles to catch some interrupts
+               tcs
+               cli                  ; fall through here -- saves a BRA instruction
+
+:outer
+               ldx   #8             ; Enable interrupts at least once every 8 lines
+               sei
+               _R1W1
+:inner         jmp   $0000
+
+:exit
+               _R0W0
+               lda   :stk_save
+               tcs
+               cli
+
+               pld
+               rts
+
+]dp            equ   158
+               lup   80             ; A full width screen is 160 bytes / 80 words
+               pei   ]dp
+]dp            equ   ]dp-2
+               --^
+:pei_end
+               jmp   :control
+
+:stk_save      ds    2
+:screen_width  ds    2
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