diff options
Diffstat (limited to 'fx/blink.s')
| -rw-r--r-- | fx/blink.s | 56 |
1 files changed, 41 insertions, 15 deletions
@@ -183,8 +183,9 @@ reset: ;; span 4 banks (that is, the first pattern table on the PPU). We have to ;; account for that when writing into the first two registers (and that's ;; why they are set to 0 and 2 respectively). Other than that, R2-R5 hold - ;; which bank goes into the other remaining regions which are 1KB each. - ;; Because of this, we can simply assign one 1KB bank to each register. + ;; which bank goes into the other remaining regions which are 1KB each + ;; (hence, the second pattern table is controlled via R2-R5). Because of + ;; this, we can simply assign one 1KB bank to each register. ;; ;; To sum things up, on this setup the first pattern table is equally ;; divided by 2 and it's controlled by R0 and R1 respectively. The second @@ -246,12 +247,28 @@ reset: ;;; NOTE: mainly as usual except that a bit of game loop has been added to ;;; handle the blinking state. .proc main + ;; The code will iterate between banks 4 and 6 on the pattern table, as + ;; that's where sprites are located (check the CHARS segment for more info). + lda #4 + sta Vars::last_bank + jsr Diskun::init_palettes jsr init_sprites cli - lda #%10010000 + + ;; NOTE: This looks like other examples, but here having background tiles on + ;; the first pattern table and sprite tiles on the second pattern table is + ;; not a matter of personal taste, but there are technical reasons for it. + ;; If you check how IRQs work on the MMC3 chip: + ;; https://www.nesdev.org/wiki/MMC3#IRQ_Specifics; you will find that on 8x8 + ;; tile mode having this arrangement actually spares us from a hardware bug. + ;; Long story short, placing background tiles first and sprite tiles second + ;; make scanline IRQs reliable. Hence, at least on all MMC3 examples, this + ;; will be guaranteed. + lda #%10001000 sta $2000 + lda #%00011110 sta $2001 @@ -277,17 +294,19 @@ reset: lda #0 sta Vars::counter - ;; The whole trick is done on R0. So select it. - lda #0 + ;; The whole trick is done on R2, which points to $1000, where the sprite + ;; tiles begin. Hence, select it. + lda #2 sta MMC3::BANK_SELECT - ;; The value for the register is either 0 or 2 depending on its last value. + ;; The value for the register is either 4 or 6 depending on its last value. lda Vars::last_bank + cmp #4 beq :+ - lda #0 - beq @set + lda #4 + jmp @set : - lda #2 + lda #6 @set: ;; Save which is the bank being used both internally and onto the MMC3 chip. sta Vars::last_bank @@ -369,14 +388,21 @@ irq: ;;; too much for this example, but it's a reasonable size for an MMC3 game. ;;; Considering only one pair of pattern tables are available at any given ;;; moment (8KB), this means that we need space for 128 / 8 = 16 pairs of -;;; pattern tables here. For this example the first 8KB are realy only used, and -;;; the rest are left with a default value ($00). +;;; pattern tables here. For this example the first 8KB are really only used, +;;; and the rest are left with a default value ($00). .segment "CHARS" -;; First 8KB (note that diskun0.chr and diskun1.chr are both 2KB long) -.incbin "../assets/diskun0.chr" ; First half of the first pattern table has the default diskun character. -.incbin "../assets/diskun1.chr" ; Second half of the first pattern table simply has the blinking version. -.res $1000, $00 ; Second pattern table is left with the default value. +;; As explained when initializing the PPUCTRL register ($2000), it's actually +;; important to place background tiles first and sprite tiles second on the +;; MMC3. This is guaranteed here by setting the first pattern table as empty (we +;; have no background on this example, really). The second pattern table is then +;; filled with `diskun0.chr` and `diskun1.chr`, which are 2KB each. Hence, the +;; 4th CHR bank contains the regular character, and the 6th CHR bank contains +;; the blinking version. You can see these values used when initializing +;; `Vars::last_bank`, or when performing bank switching. +.res $1000, $00 +.incbin "../assets/diskun0.chr" ; First half of the second pattern table has the default diskun character. +.incbin "../assets/diskun1.chr" ; Second half of the second pattern table simply has the blinking version. ;; The 15 other 8KB portions are left empty. .res $2000, $00 |