mirror of
https://github.com/sehugg/8bitworkshop.git
synced 2024-11-24 12:31:25 +00:00
126 lines
3.1 KiB
Plaintext
126 lines
3.1 KiB
Plaintext
|
|
processor 6502
|
|
include "vcs.h"
|
|
include "macro.h"
|
|
|
|
org $f000
|
|
|
|
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
|
;
|
|
; We're going to use a more clever way to position sprites
|
|
; ("players") which relies on additional TIA features.
|
|
; Because the CPU timing is 3 times as coarse as the TIA's,
|
|
; we can only access 1 out of 3 possible positions using
|
|
; CPU delays alone.
|
|
; Additional TIA registers let us nudge the final position
|
|
; by discrete TIA clocks and thus target all 160 positions.
|
|
;
|
|
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
|
|
|
|
counter equ $81
|
|
|
|
start CLEAN_START
|
|
|
|
nextframe
|
|
VERTICAL_SYNC
|
|
|
|
; 34 lines of VBLANK
|
|
ldx #34
|
|
lvblank sta WSYNC
|
|
dex
|
|
bne lvblank
|
|
|
|
; Instead of representing the horizontal position in CPU clocks,
|
|
; we're going to use TIA clocks.
|
|
|
|
lda counter ; load the counter as horizontal position
|
|
and #$7f ; force range to (0-127)
|
|
|
|
; We're going to divide the horizontal position by 15.
|
|
; The easy way on the 6502 is to subtract in a loop.
|
|
; Note that this also conveniently adds 5 CPU cycles
|
|
; (15 TIA clocks) per iteration.
|
|
sta WSYNC ; 35th line
|
|
sta HMCLR ; reset the old horizontal position
|
|
DivideLoop
|
|
sbc #15 ; subtract 15
|
|
bcs DivideLoop ; branch until negative
|
|
; A now contains (the remainder - 15).
|
|
; We'll convert that into a fine adjustment, which has
|
|
; the range -8 to +7.
|
|
eor #7
|
|
asl ; HMOVE only uses the top 4 bits, so shift by 4
|
|
asl
|
|
asl
|
|
asl
|
|
; The fine offset goes into HMP0
|
|
sta HMP0
|
|
; Now let's fix the coarse position of the player, which as you
|
|
; remember is solely based on timing. If you rearrange any of the
|
|
; previous instructions, position 0 won't be exactly on the left side.
|
|
sta RESP0
|
|
; Finally we'll do a WSYNC followed by HMOVE to apply the fine offset.
|
|
sta WSYNC ; 36th line
|
|
sta HMOVE ; apply offset
|
|
|
|
; We'll see this method again, and it can be made into a subroutine
|
|
; that works on multiple objects.
|
|
|
|
; Now draw the 192 scanlines, drawing the sprite.
|
|
; We've already set its horizontal position for the entire frame,
|
|
; but we'll try to draw something real this time, some digits
|
|
; lifted from another game.
|
|
ldx #192
|
|
lda #0 ; changes every scanline
|
|
ldy #0 ; sprite data index
|
|
lvscan
|
|
sta WSYNC ; wait for next scanline
|
|
sty COLUBK ; set the background color
|
|
lda NUMBERS,y
|
|
sta GRP0 ; set sprite 0 pixels
|
|
iny
|
|
cpy #60
|
|
bcc wrap1
|
|
ldy #0
|
|
wrap1
|
|
dex
|
|
bne lvscan
|
|
|
|
; Clear the background color and sprites before overscan
|
|
stx COLUBK
|
|
stx GRP0
|
|
; 30 lines of overscan
|
|
ldx #30
|
|
lvover sta WSYNC
|
|
dex
|
|
bne lvover
|
|
|
|
; Cycle the sprite colors for the next frame
|
|
inc counter
|
|
lda counter
|
|
sta COLUP0
|
|
jmp nextframe
|
|
|
|
; Bitmap pattern for digits
|
|
|
|
NUMBERS ;;{w:8,h:6,count:10,brev:1};;
|
|
.byte $EE,$AA,$AA,$AA,$EE,$00
|
|
.byte $22,$22,$22,$22,$22,$00
|
|
.byte $EE,$22,$EE,$88,$EE,$00
|
|
.byte $EE,$22,$66,$22,$EE,$00
|
|
.byte $AA,$AA,$EE,$22,$22,$00
|
|
.byte $EE,$88,$EE,$22,$EE,$00
|
|
.byte $EE,$88,$EE,$AA,$EE,$00
|
|
.byte $EE,$22,$22,$22,$22,$00
|
|
.byte $EE,$AA,$EE,$AA,$EE,$00
|
|
.byte $EE,$AA,$EE,$22,$EE,$00
|
|
;; end
|
|
|
|
; Epilogue
|
|
org $fffc
|
|
.word start
|
|
.word start
|
|
|
|
; QUESTION: What if you don't set the fine offset?
|
|
; QUESTION: What if you don't set the coarse offset?
|