Step 2 - Timers
Entry posted by SpiceWare
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In Step 1 I used loops of sta WSYNC commands to delay the program so that Vertical Blank and OverScan would last for the proper duration. That method works fine when all we want to do is generate a static display, but as soon as we start to add game logic that won't work out so well.
The problem with the game logic is there will be so many different paths the code can take that it is nearly impossible for us to know how long the code ran, and thus we won't know how many scanlines we need to delay before the next section of code can run. As an example, if the player isn't moving the joystick then none of the "move player" logic will run. If the player is moving the joystick left and up then the "move horizontal" and "move vertical" logic will run. If the player is only holding the joystick left then only the "move horizontal" logic will run.
Fortunately for us, the Atari 2600 contains a RIOT chip. That acronym stands for RAM, Input/Output and Timer. We're interested in the Timer for this update to Collect, we'll look at RAM and I/O in a later update.
First thing I changed was OverScan. The original routine looked like this:
OverScan:
sta WSYNC ; Wait for SYNC (halts CPU until end of scanline)
lda #2 ; LoaD Accumulator with 2 so D1=1
sta VBLANK ; STore Accumulator to VBLANK, D1=1 turns image output off
ldx #27 ; LoaD X with 27
osLoop:
sta WSYNC ; Wait for SYNC (halts CPU until end of scanline)
dex ; DEcrement X by 1
bne osLoop ; Branch if Not Equal to 0
rts ; ReTurn from Subroutine
So what we want to do is set a timer that will go off after 27 scanlines to pass. There's 76 cycles of time per scanline, so we need the timer to go off after 2052 cycles have passed. When we set the timer, we also select how frequently the timer will decrement in value. RIOT has options to decrement the timer every 1, 8, 64 or 1024 cycles.
The timer is set using a single byte, so it can only be set to any value from 0 to 255. As such, we know we can't use decrement every 1 cycle as 2052 is too large. So let's check if decrement every 8 cycles will work:
2052/8 = 256.5
Almost, but 256 won't fit so we're going to have to use the decrement every 64 cycles option. To figure out the initial value to set the timer to, use this equation:
(scanlines * 76) / 64
The new OverScan routine that uses the timer looks like this:
OverScan:
sta WSYNC ; Wait for SYNC (halts CPU until end of scanline)
lda #2 ; LoaD Accumulator with 2 so D1=1
sta VBLANK ; STore Accumulator to VBLANK, D1=1 turns image output off
lda #32 ; set timer for 27 scanlines, 32 = ((27 * 76) / 64)
sta TIM64T ; set timer to go off in 27 scanlines
; game logic will go here
OSwait:
sta WSYNC ; Wait for SYNC (halts CPU until end of scanline)
lda INTIM ; Check the timer
bne OSwait ; Branch if its Not Equal to 0
rts ; ReTurn from Subroutine
For Vertical Blank we're going to set up the timer a little different. There's time in VerticalSync we can utilize, so we'll set the timer there - look for the code using ldx and stx:
VerticalSync:
lda #2 ; LoaD Accumulator with 2 so D1=1
ldx #49 ; LoaD X with 49
sta WSYNC ; Wait for SYNC (halts CPU until end of scanline)
sta VSYNC ; Accumulator D1=1, turns on Vertical Sync signal
stx TIM64T ; set timer to go off in 41 scanlines (49 * 64) / 76
sta WSYNC ; Wait for Sync - halts CPU until end of 1st scanline of VSYNC
sta WSYNC ; wait until end of 2nd scanline of VSYNC
lda #0 ; LoaD Accumulator with 0 so D1=0
sta WSYNC ; wait until end of 3rd scanline of VSYNC
sta VSYNC ; Accumulator D1=0, turns off Vertical Sync signal
rts ; ReTurn from Subroutine
We're also going to check the timer in the Kernel so we can start drawing the screen as soon as it goes off:
Kernel:
sta WSYNC ; Wait for SYNC (halts CPU until end of scanline)
lda INTIM ; check the timer
bne Kernel ; Branch if its Not Equal to 0
; turn on the display
sta VBLANK ; Accumulator D1=0, turns off Vertical Blank signal (image output on)
; draw the screen
ldx #192 ; Load X with 192
KernelLoop:
sta WSYNC ; Wait for SYNC (halts CPU until end of scanline)
stx COLUBK ; STore X into TIA's background color register
dex ; DEcrement X by 1
bne KernelLoop ; Branch if Not Equal to 0
rts ; ReTurn from Subroutine
For the moment, these changes leave VerticalBlank with nothing to do:
VerticalBlank:
rts ; ReTurn from Subroutine
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