118 lines
4.6 KiB
ArmAsm
118 lines
4.6 KiB
ArmAsm
; Implementation of a PEI Slammer that updates a rectangular screen area. The only tweak that
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; this implementation does is that it does break up the slam into chunks of scan lines to allow
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; time for interrupts to be serviced in a timely manner.
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;
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; This is a fairly basic slam in that it does not try to align the direct page. To enhance the
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; slammer, note that page-aligned addresses repeat every 8 scan lines and some lines would need
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; to be split into two slams to keep the direct page aligned.
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;
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; At best, this saves 1 cycles per word, or 80 cycles for a full scanline -- which is only about
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; 12 additional instructions, so this is an optimization that is unlikely to lead to a net
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; improvement.
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;
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; X = first line (inclusive), valid range of 0 to 199
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; Y = last line (exclusive), valid range >X up to 200
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_PEISlam
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cpx #200
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bcc *+3
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rts
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cpy #201
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bcc *+3
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rts
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txa
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stal :screen_width_1
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tya
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cmpl :screen_width_1
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bcs *+3
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rts
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lda ScreenWidth
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dec
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stal :screen_width_1 ; save the width-1 outside of the direct page
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lda #:pei_end ; patch the PEI entry address
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sec
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sbc ScreenWidth
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stal :inner+1
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phx
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tya
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sec
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sbc 1,s
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ply
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tay ; get the number of lines in the y register
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txa
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asl
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tax
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lda RTable,x ; This is the right visible byte, so add one to get the
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tax ; left visible byte (cache in x-reg)
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sec
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sbc ScreenWidth
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inc
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phd ; save the current direct page and assign the base
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tcd ; screen address to the direct page register
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tsc
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stal :stk_save ; save the stack pointer to restore later
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clc ; clear before the loop -- nothing in the loop affect the carry bit
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brl :outer ; hop into the entry point.
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]dp equ 158
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lup 80 ; A full width screen is 160 bytes / 80 words
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pei ]dp
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]dp equ ]dp-2
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--^
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:pei_end
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tdc ; Move to the next line
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adc #160
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tcd
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adcl :screen_width_1
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tcs
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dey ; decrement the total counter, if zero then we're done
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beq :exit
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dex ; decrement the inner counter. Both counters are set
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beq :restore ; up so that they fall-through by default to save a cycle
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; per loop iteration.
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:inner jmp $0000 ; 25 cycles of overhead per line. A full width slam executes all
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; 80 of the PEI instructions which we expect to take 7 cycles
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; since the direct page is not aligned. So total overhead is
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; 25 / (25 + 7 * 80) = 4.27% of execution
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;
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; Without the interrupt breaks, we could remove the dex/beq test
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; and save 4 cycles per loop which takes the overhead down to
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; only 3.6%
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:restore
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tsx ; save the current stack
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_R0W0 ; restore the execution environment and
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ldal :stk_save ; give a few cycles to catch some interrupts
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tcs
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cli ; fall through here -- saves a BRA instruction
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:outer
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sei
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txs ; set the stack address to the right edge
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ldx #8 ; Enable interrupts at least once every 8 lines
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_R1W1
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bra :inner
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:exit
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_R0W0
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ldal :stk_save
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tcs
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cli
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pld
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rts
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:stk_save ds 2
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:screen_width_1 ds 2
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