dos33fsprogs/space_bars/title.s
Vince Weaver 757f9d9c3d space_bars: some initial cycle-counting
it would help if I could subtract
2018-10-06 20:35:28 -04:00

186 lines
3.5 KiB
ArmAsm

;================================
; Show the title screen
; return when a key is pressed
;================================
title_screen:
;===================
; init screen
jsr TEXT
jsr HOME
bit KEYRESET
;===================
; init vars
lda #0
sta DRAW_PAGE
;=============================
; Load graphic page0
lda #$0c
sta BASH
lda #$00
sta BASL ; load image to $c00
lda #<spacebars_title_low
sta GBASL
lda #>spacebars_title_low
sta GBASH
jsr load_rle_gr
lda #4
sta DRAW_PAGE
jsr gr_copy_to_current ; copy to page1
; GR part
bit PAGE1
bit LORES ; 4
bit SET_GR ; 4
bit FULLGR ; 4
;=============================
; Load graphic page1
lda #$0c
sta BASH
lda #$00
sta BASL ; load image to $c00
lda #<spacebars_title_high
sta GBASL
lda #>spacebars_title_high
sta GBASH
jsr load_rle_gr
lda #0
sta DRAW_PAGE
; GR part
bit PAGE0
;==============================
; setup graphics for vapor lock
;==============================
jsr vapor_lock
; found first line of black after green, at up to line 26 on screen
; so we want roughly 22 lines * 4 = 88*65 = 5720 + 4550 = 10270
; - 65 (for the scanline we missed) = 10205 - 12 = 10193
jsr gr_copy_to_current ; 6+ 9292
; 10193 - 9298 = 895
; Fudge factor (unknown) -30 = 865
; GR part
bit LORES ; 4
bit SET_GR ; 4
bit FULLGR ; 4
; Try X=88 Y=2 cycles=893 R2
nop
ldy #2 ; 2
loopA:
ldx #88 ; 2
loopB:
dex ; 2
bne loopB ; 2nt/3
dey ; 2
bne loopA ; 2nt/3
jmp display_loop
.align $100
;================================================
; Display Loop
;================================================
; each scan line 65 cycles
; 1 cycle each byte (40cycles) + 25 for horizontal
; Total of 12480 cycles to draw screen
; Vertical blank = 4550 cycles (70 scan lines)
; Total of 17030 cycles to get back to where was
; We want to alternate between page1 and page2 every 65 cycles
; vblank = 4550 cycles to do scrolling
; 2 + 48*( (4+2+25*(2+3)) + (4+2+23*(2+3)+4+5)) + 9)
; 48*[(6+125)-1] + [(6+115+10)-1]
display_loop:
ldy #96 ; 2
outer_loop:
bit PAGE0 ; 4
ldx #12 ; 65 cycles with PAGE0 ; 2
page0_loop: ; delay 61+bit
dex ; 2
bne page0_loop ; 2/3
; bit(4) -1(fallthrough) + loop*5 -1(fallthrouh)+4 extra = 61
; 5L = 55
bit PAGE1 ; 4
ldx #11 ; 65 cycles with PAGE1 ; 2
;
page1_loop: ; delay 115+(7 loop)+4 (bit)+4(extra)
dex ; 2
bne page1_loop ; 2/3
dey ; 2
bne outer_loop ; 2/3
;======================================================
; We have 4550 cycles in the vblank, use them wisely
;======================================================
; do_nothing should be 4550+1 -2-9 -7= 4533
jsr do_nothing ; 6
lda KEYPRESS ; 4
bpl no_keypress ; 3
jmp return
no_keypress:
jmp display_loop ; 3
;=================================
; do nothing
;=================================
; and take 4533-6 = 4527 cycles to do it
do_nothing:
; Try X=4 Y=174 cycles=4525 R2
nop ; 2
ldy #174 ; 2
loop1:
ldx #4 ; 2
loop2:
dex ; 2
bne loop2 ; 2nt/3
dey ; 2
bne loop1 ; 2nt/3
return:
rts ; 6