Introduction
In this blog post, I will endeavor to enhance the performance of the provided code by optimizing its execution time. Furthermore, I will also introduce modifications to vary the displayed colours.
The following code fills the emulator's bitmapped display with the colour yellow.
lda #$00 ; set a pointer in memory location $40 to point to $0200
sta $40 ; ... low byte ($00) goes in address $40
lda #$02
sta $41 ; ... high byte ($02) goes into address $41
lda #$07 ; colour number
ldy #$00 ; set index to 0
loop:
sta ($40),y ; set pixel colour at the address (pointer)+Y
iny ; increment index
bne loop ; continue until done the page (256 pixels)
inc $41 ; increment the page
ldx $41 ; get the current page number
cpx #$06 ; compare with 6
bne loop ; continue until done all pages
Calculating Performance
Q. Execution time, assuming a 1 MHz clock speed and total memory usage
https://www.masswerk.at/6502/6502_instruction_set.html#STA
lda #$00 ; 2 cycles
sta $40 ; 3 cycles
lda #$02 ; 2 cycles
sta $41 ; 3 cycles
lda #$07 ; 2 cycles
ldy #$00 ; 2 cycles
loop:
sta ($40),y ; 6 cycles
iny ; 2 cycles
bne loop ; 3 cycles
inc $41 ; 5 cycles
ldx $41 ; 4 cycles
cpx #$06 ; 2 cycles
bne loop ; 3 cycles
- Total cycle count = 39 cycles.
- Total time = 39 cycles / 1,000,000 cycles per sec = 0.000039 sec or 39 microsecond.
- Total memory usage = 13 instructions * 1 byte + 2 variables * 2 bytes = 17 bytes
Q. Optimized version in terms of faster execution time
ldx #$00 ; Set pointer in X register
stx $40 ; Store low byte in address $40
inx ; Increment X
stx $41 ; Store high byte in address $41
lda #$07 ; Colour number
ldy #$00 ; Set index to 0
ldx $41 ; Get the current page number (moved outside the loop)
loop:
sta ($40),y ; Set pixel colour at the address (pointer)+Y
iny ; Increment index
bne loop ; Continue until done the page (256 pixels)
inc $41 ; Increment the page
cpx #$06 ; Compare with 6
bne loop ; Continue until done all pages
- Total cycle count = 35 cycles.
- Total time = 35 cycles / 1,000,000 cycles per sec = 0.000035 sec or 35 microsecond.
Modifying the Code
Q. Modified code to fill the display with light blue instead of yellow.
ldx #$00 ; Set pointer in X register
stx $40 ; Store low byte in address $40
inx ; Increment X
stx $41 ; Store high byte in address $41
lda #$0E ; Light blue color number (instead of #$07 for yellow)
ldy #$00 ; Set index to 0
ldx $41 ; Get the current page number (moved outside the loop)
loop:
sta ($40),y ; Set pixel colour at the address (pointer)+Y
iny ; Increment index
bne loop ; Continue until done the page (256 pixels)
inc $41 ; Increment the page
cpx #$06 ; Compare with 6
bne loop ; Continue until done all pages
Q. Modified code to fill the display with a different colour on each page (each "page" will be one-quarter of the bitmapped display).
ldx #$00 ; Set pointer in X register
stx $40 ; Store low byte in address $40
inx ; Increment X
stx $41 ; Store high byte in address $41
ldy #$00 ; Set index to 0
ldx $41 ; Get the current page number (moved outside the loop)
loop:
; Calculate color based on page number
lda $41 ; Load current page number
asl ; Multiply by 2 (each page is one-quarter)
clc
adc #$0E ; Add base color value
sta ($40),y ; Set pixel colour at the address (pointer)+Y
iny ; Increment index
bne loop ; Continue until done the page (256 pixels)
inc $41 ; Increment the page
cpx #$06 ; Compare with 6
bne loop ; Continue until done all pages
My thoughts through the process were...
My first impression of assembly language is worrying about not understanding anything in the given code. I still need a lot of time to figure out what is going on with the code. While I was calculating performance and modifying the codes to improve the performance, I learned how to calculate execution time with the total cycle count. It was very cumbersome to find each cycle and count to figure out the execution time, though.
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