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combine two pseudo-random methods
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@donno2048
donno2048 authored Dec 17, 2024
1 parent 2225272 commit 46553d9
Showing 1 changed file with 33 additions and 34 deletions.
67 changes: 33 additions & 34 deletions snake.asm
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; SI: 0x7D0, screen size (40x25x2 bytes), used in food generation, edge checks, also used for screen accesses but constantly reinitialized
; DI: pointer to memory location where the current position of the snake head is stored
; SP: pointer to memory location where the current position of the snake tail is stored
lds di, [BYTE bx+si+0x0] ; SI=0x100 and BX=0x0 at program start, this initializes DS and DI and the 0 serves as a dummy byte for the LDS itself as machine code at 0x100 is 0xC5,0x78,0x00,0xB8
start: ; reset game
mov ax, 0x200A ; first byte of the instruction is 0xB8 used for LDS, also sets AX to the initial value for DX to swap later: DH set to empty space, DL to 0xA, DX satisfies DX*8%0x10000=0x50 so good for the wall building
cwd ; set DX to 0x0 to swap later with AX to set video mode (AH=0x0) to mode 0 (AL=0x0), text mode 40x25 16 colors
xchg ax, dx ; swap AX and DX
int 0x10 ; set video mode using BIOS interrupt call, also clears the screen
mov sp, di ; set SP to current head pointer
mov bx, [si] ; reset head position, SI always points to a valid screen position containing 0x720 after setting video mode, also SBB when jumping to .food-1
.food: ; create new food item, always jumping to .food-1 for randomizing with SBB
lahf ; read FLAGS into AH for randomization after restart and `SBB AL, 0x9F` for the next food generations
and si, ax ; mask with SI to make divisible by 4 and less than or equal to screen size
add [si], cl ; place food item and check if position was empty by applying ADD with CL (assumed to be 0xFF)
.input: ; handle keyboard input
mov si, 0x7D0 ; initialize SI
js .food-1 ; if position was occupied by snake or wall in food generation => try again, if we came from main loop SF=0
in al, 0x60 ; read scancode from keyboard controller - bit 7 is set in case key was released
imul dl ; we want to map scancodes for arrow up (0x48/0xC8), left (0x4B/0xCB), right (0x4D/0xCD), down (0x50/0xD0) to movement offsets
aam 0x14 ; IMUL (AH is irrelevant here), AAM and AAD with some magic constants maps up => -80, left => -2, right => 2, down => 80
aad 0x44 ; using arithmetic instructions is more compact than checks and conditional jumps
cbw ; but causes weird snake movements though with other keys
add ax, bx ; set AX to new head position
cmp ax, si ; check if head crossed vertical edge by comparing against SI
stosw ; store head position (SS=ES by default) and advance head pointer
xchg ax, bx ; save new head position to BX
rcr BYTE [bx], 0x1 ; RCR head position to set snake character
jno start ; if it already had snake or wall in it or if it crossed a vertical edge, OF=0 from RCR => game over
jc .food-1 ; if food was consumed, CF=1 from RCR => generate new food
.wall: ; draw an invisible wall on the left side
sub si, dx ; go one line and 0x2000 bytes backwards (the added bytes wrap nicely as 0x10000%0x2000=0)
mov [si], cl ; store wall character
jnz .wall ; jump to draw the next wall
pop si ; no food was consumed so pop tail position into SI
mov [si], dh ; clear old tail position on screen
jns .input ; loop to keyboard input, SF=0 from SUB
lds di, [BYTE bx+si+0x0] ; SI=0x100 and BX=0x0 at program start, this initializes DS and DI and the 0 serves as a dummy byte for the LDS itself as machine code at 0x100 is 0xC5,0x78,0x00,0xB8
start: ; reset game
mov ax, 0x200A ; first byte of the instruction is 0xB8 used for LDS, also sets AX to the initial value for DX to swap later: DH set to empty space, DL to 0xA, DX satisfies DX*8%0x10000=0x50 so good for the wall building
cwd ; set DX to 0x0 to swap later with AX to set video mode (AH=0x0) to mode 0 (AL=0x0), text mode 40x25 16 colors
xchg ax, dx ; swap AX and DX
int 0x10 ; set video mode using BIOS interrupt call, also clears the screen
mov sp, di ; set SP to current head pointer
lodsw ; set AX to 0x720 and advance SI so that after `SUB SI, AX` the SI register will contain a valid food position
mov bx, [BYTE bx+si+0x50] ; reset head position, the address always points to a valid screen position containing 0x720 after setting video mode, also `POP AX`, `PUSH AX` when jumping to .food-2
.food: ; create new food item, always jumping to .food-2 for doing the `POP & PUSH`
sub si, ax ; make the food position a little more random
add [si], cl ; place food item by applying ADD with CL
.input: ; handle keyboard input
mov si, 0x7D0 ; initialize SI
in al, 0x60 ; read scancode from keyboard controller - bit 7 is set in case key was released
imul dl ; we want to map scancodes for arrow up (0x48/0xC8), left (0x4B/0xCB), right (0x4D/0xCD), down (0x50/0xD0) to movement offsets
aam 0x14 ; IMUL (AH is irrelevant here), AAM and AAD with some magic constants maps up => -80, left => -2, right => 2, down => 80
aad 0x44 ; using arithmetic instructions is more compact than checks and conditional jumps
cbw ; but causes weird snake movements though with other keys
add ax, bx ; set AX to new head position
cmp ax, si ; check if head crossed vertical edge by comparing against SI
stosw ; store head position (SS=ES by default) and advance head pointer
xchg ax, bx ; save new head position to BX
rcr BYTE [bx], 0x1 ; RCR head position to set snake character
jno start ; if it already had snake or wall in it or if it crossed a vertical edge, OF=0 from RCR => game over
jc .food-2 ; if food was consumed, CF=1 from RCR => generate new food
.wall: ; draw an invisible wall on the left side
sub si, dx ; go one line and 0x2000 bytes backwards (the added bytes wrap nicely as 0x10000%0x2000=0)
mov [si], cl ; store wall character
jnz .wall ; jump to draw the next wall
pop si ; no food was consumed so pop tail position into SI
mov [si], dh ; clear old tail position on screen
jns .input ; loop to keyboard input, SF=0 from SUB

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