I am playing around with 6502 assembler here: http://skilldrick.github.io/easy6502
I have made this that just puts a random color pixel in a random place on the screen:
LDY #$00 ; Clear Y
loop:
JSR genPos ; Put new position in memory
LDA $fe ; Get random number for color
STA ($10), Y ; Put pixel on screen
JMP loop
genPos:
STA $10 ; Store accumulator in low
LDA $fe ; Get new random number (WHY, oh why?)
AND #$03 ; Mask out low two bits (=numbers 0-3)
CLC ; Clear carry flag
ADC #2 ; Add 2 (= numbers 2-5)
STA $11 ; Store number in high
RTS
I am trying to use as few instructions as possible. My problem is that if I don't put an extra LDA $fe in the genPos sub routine the pixels are drawn in a very strange pattern where if I do have the extra LDA the code works perfectly. I can't comprehend why - can anyone give me a hint?
Regards, Jacob
It already is making a fine low byte! This line:
LDA $fe ; Get new random number (WHY, oh why?)
goes on to decide the high byte, and if you don't generate a new random number, the y value will be dependent on the lowest two bits of the x value, causing the diagonals you're seeing: the value of x & 3 always equals which segment of screen the value is being drawn on, meaning you get a pattern like
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█ █ █ █ █ █ █ █ | x & 3 == 0 in $200-$2FF
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█ █ █ █ █ █ █ █ | x & 3 == 1 in $300-$3FF
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█ █ █ █ █ █ █ █ | x & 3 == 2 in $400-$4FF
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█ █ █ █ █ █ █ █ | x & 3 == 3 in $500-$5FF
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