; step3.asm — the MSX tutorial (docs/msx/TUTORIAL.md), step 3: the snake — moving, steering, crashing. ; ; npm run msx:asm -- docs/msx/snake/step3.asm makes docs/msx/snake/step3.rom (in the repository) ; node z80asm.mjs step3.asm the same, with the assembler from eeditor.app ; --------------------------------------------------------------------------- ; The BIOS: routines in the MSX's ROM that every cartridge can call ; --------------------------------------------------------------------------- CHGCLR equ 0x0062 ; set the screen's colours from FORCLR, BAKCLR and BDRCLR FILVRM equ 0x0056 ; fill BC bytes of video memory from HL with A LDIRVM equ 0x005C ; copy BC bytes from HL in memory to DE in video memory INIT32 equ 0x006F ; SCREEN 1: 32×24 tiles of 8×8 pixels WRTPSG equ 0x0093 ; write E into sound-chip register A GTSTCK equ 0x00D5 ; with A=0: the cursor keys, as a direction 0–8 GTTRIG equ 0x00D8 ; with A=0: the space bar — 0xFF while it's held, else 0 FORCLR equ 0xF3E9 ; the BIOS's colours: text, BAKCLR equ 0xF3EA ; background, BDRCLR equ 0xF3EB ; and border ; --------------------------------------------------------------------------- ; The video chip, in SCREEN 1 ; --------------------------------------------------------------------------- VDP_DATA equ 0x98 ; read or write video memory, the address moving on each time VDP_CTRL equ 0x99 ; set that address (two bytes, low first) PATTERNS equ 0x0000 ; 256 tiles × 8 bytes: each byte a row of 8 pixels NAMES equ 0x1800 ; 32×24 bytes: the tile on each cell of the screen COLOURS equ 0x2000 ; 32 bytes: the colours of each group of 8 tiles (ink × 16 + paper) ; our tiles: the letters' tiles are the font; these four are drawn below, each the first of ; a group of eight so that each can have its own colours EMPTY equ ' ' WALL equ 0x80 BODY equ 0x88 HEAD equ 0x90 APPLE equ 0x98 ; --------------------------------------------------------------------------- ; Our variables, in RAM ; --------------------------------------------------------------------------- RING equ 0xE000 ; the snake: 256 cells of 2 bytes, a ring — the head at HEADI, HEADI equ 0xE200 ; the tail at TAILI; each index wraps from 255 to 0 TAILI equ 0xE201 DIR equ 0xE202 ; 2 bytes: what to add to a cell to go a step (1, -1, 32 or -32) WANT equ 0xE204 ; 2 bytes: the direction the keys asked for GROW equ 0xE206 ; how many more steps the tail stays put TIMER equ 0xE207 ; frames to the next step SPEED equ 0xE208 ; frames between steps EATEN equ 0xE209 ; apples since the last speed-up SCORE equ 0xE20A ; 2 bytes, 4 decimal digits (BCD: a digit a nibble) HISCORE equ 0xE20C ; 2 bytes, the same SEED equ 0xE20E ; 2 bytes, for the random numbers ; --------------------------------------------------------------------------- ; The cartridge's header: the BIOS looks for "AB" at 0x4000 and calls the address after it ; --------------------------------------------------------------------------- org 0x4000 db "AB" dw start ds 12 ; the rest of the header: unused start: ld a,15 ; white letters ld (FORCLR),a ld a,1 ; on black, black border ld (BAKCLR),a ld (BDRCLR),a call CHGCLR call INIT32 ; SCREEN 1, the font loaded ; our four tiles' pixels, each at the start of its group of eight ld hl,tiles ld de,PATTERNS + WALL*8 ld b,4 .tile push bc push hl push de ld bc,8 call LDIRVM pop hl ; the video address… ld bc,8*8 add hl,bc ; …eight tiles on ex de,hl pop hl ; the pixels… ld bc,8 add hl,bc ; …the next tile's pop bc djnz .tile ; and their colours: groups 16 to 19 ld hl,colours ld de,COLOURS + WALL/8 ld bc,4 call LDIRVM ld hl,0 ; the best score stays 0000 until step 5 ld (HISCORE),hl ; --------------------------------------------------------------------------- ; A game ; --------------------------------------------------------------------------- newgame: ld hl,0 ld (SCORE),hl call field ; the snake: four cells in the middle row, the head on the right xor a ld (TAILI),a ld a,3 ld (HEADI),a ld hl,12*32 + 8 ld ix,RING ld b,4 .body ld (ix+0),l ld (ix+1),h ld a,BODY call put inc hl inc ix inc ix djnz .body dec hl ld a,HEAD call put ld hl,1 ; going right ld (DIR),hl ld (WANT),hl ld a,8 ld (SPEED),a ld (TIMER),a play: ei halt ; wait for the next frame: 60 a second call steer ld hl,TIMER dec (hl) jr nz,play ; not time to move yet ld a,(SPEED) ld (TIMER),a call step jr nc,play ; no carry: still alive ; crashed ld hl,txt_over ld de,11*32 + 11 call print .end ei halt jr .end ; The cursor keys: remember the direction they point, for the next step. steer: xor a call GTSTCK ; 1 up, 3 right, 5 down, 7 left (2, 4, 6, 8 diagonals) ld hl,-32 cp 1 jr z,.want ld hl,1 cp 3 jr z,.want ld hl,32 cp 5 jr z,.want ld hl,-1 cp 7 ret nz .want ld (WANT),hl ret ; One step of the snake. Carry set: it crashed. step: ld hl,(WANT) ; turn — unless it means going straight back ld de,(DIR) add hl,de ; a direction and its opposite add up to 0 ld a,h or l jr z,.ahead ld hl,(WANT) ld (DIR),hl .ahead ld a,(HEADI) ; the head's cell call ringat ld e,(hl) inc hl ld d,(hl) ex de,hl ld a,BODY ; becomes body call put ld de,(DIR) add hl,de ; the next cell call get cp EMPTY jr z,.move or a ; a wall, or itself: crashed sbc hl,de ; back to where the head was ld a,HEAD call put scf ret .move ld a,HEAD call put ex de,hl ; DE: the new head's cell ld a,(HEADI) inc a ld (HEADI),a call ringat ld (hl),e inc hl ld (hl),d ld a,(TAILI) ; the tail moves up a cell call ringat ld e,(hl) inc hl ld d,(hl) ex de,hl ld a,EMPTY call put ld hl,TAILI inc (hl) or a ; carry clear: alive ret ; --------------------------------------------------------------------------- ; The screen ; --------------------------------------------------------------------------- ; The playing field: the score line, a wall round the rest. field: ld hl,NAMES ld bc,32*24 ld a,EMPTY call FILVRM ld hl,NAMES + 1*32 ; the top wall ld bc,32 ld a,WALL call FILVRM ld hl,NAMES + 23*32 ; the bottom wall ld bc,32 ld a,WALL call FILVRM ld hl,2*32 ; the sides, rows 2 to 22 ld b,21 ld de,31 .side ld a,WALL call put ; the left… add hl,de call put ; …and the right inc hl djnz .side ld hl,txt_score ld de,1 call print ld hl,txt_hi ld de,24 call print ; and on into showscore ; The score and the best, from their BCD. showscore: ld hl,7 ld de,SCORE call bcd ld hl,27 ld de,HISCORE ; and on into bcd ; The four digits of the BCD number at DE, at cell HL. bcd: ld a,h add a,HIGH(NAMES) ld h,a call setwrite inc de ; the high byte first: the thousands and hundreds ld a,(de) call .two dec de ld a,(de) ; then the tens and units .two push af rrca rrca rrca rrca and 15 add a,'0' out (VDP_DATA),a pop af and 15 add a,'0' out (VDP_DATA),a ret ; The text at HL (ending in 0) on the screen from cell DE. print: push hl ex de,hl ld a,h add a,HIGH(NAMES) ld h,a call setwrite pop hl .next ld a,(hl) or a ret z out (VDP_DATA),a inc hl jr .next ; Tile A on cell HL (0–767). Keeps every register. put: push hl push af ld a,h add a,HIGH(NAMES) ld h,a call setwrite pop af out (VDP_DATA),a pop hl ret ; The tile on cell HL into A. Keeps the other registers. get: push hl ld a,h add a,HIGH(NAMES) ld h,a call setread pop hl in a,(VDP_DATA) ret ; Point the video chip at address HL, to write there (setwrite) or read (setread). The two ; bytes go with interrupts off: the BIOS's frame interrupt talks to the chip too. setwrite: ld a,h or 0x40 ; 01 in the top bits: for writing jr setaddr setread: ld a,h and 0x3F ; 00: for reading setaddr: di push af ld a,l out (VDP_CTRL),a pop af out (VDP_CTRL),a ei ret ; --------------------------------------------------------------------------- ; Odds and ends ; --------------------------------------------------------------------------- ; HL: the entry for index A in the ring. Keeps DE. ringat: push de ld l,a ld h,0 add hl,hl ld de,RING add hl,de pop de ret ; --------------------------------------------------------------------------- ; Data ; --------------------------------------------------------------------------- tiles: db 0b11101111 ; the wall: bricks db 0b11101111 db 0b11101111 db 0b00000000 db 0b11111110 db 0b11111110 db 0b11111110 db 0b00000000 db 0b00111100 ; the body: a round segment db 0b01111110 db 0b11111111 db 0b11111111 db 0b11111111 db 0b11111111 db 0b01111110 db 0b00111100 db 0b00111100 ; the head: with eyes db 0b01111110 db 0b11011011 db 0b11011011 db 0b11111111 db 0b11111111 db 0b01111110 db 0b00111100 db 0b00001000 ; an apple db 0b00010000 db 0b01101100 db 0b11111110 db 0b11111110 db 0b11111110 db 0b01111100 db 0b00101000 colours: db 0x6E ; wall: dark red bricks, grey mortar db 0x21 ; body: green on black db 0x31 ; head: light green db 0x81 ; apple: red txt_over: db "GAME OVER", 0 txt_score: db "SCORE", 0 txt_hi: db "HI", 0 ds 0x8000 - $, 0xFF ; fill the cartridge out to 16 KB