mov sp, ORG
mov bp, ORG
-; clear screen ----------------------------------------------------------------
+; clear direction flag
+cld
; disable cursor
mov ch, 0x3f
mov ah, 0x01
int 0x10
-; clear video memory
-mov cx, COLS * ROWS
-mov ax, VGA >> 4
-mov es, ax
-xor di, di
-mov ax, (PRINT_COLOR << 8) | DEAD
-
-rep stosw ; fill cx words at es:di with ax
-
-; setup -----------------------------------------------------------------------
-
-setup:
-
; initialize xss
mov ah, 0x00 ; get
int 0x1a ; system time
mov [XSS], dx ; cx:dx = number of clock ticks since midnight
-; initialize grid
-mov di, GRID
-mov cx, COLS * ROWS
-
-.initgrid: ; do {
- call xs ; ax = rand
- mov dl, DEAD ; dl = DEAD (likely)
-
- test ax, 0b11
-
- jnz .initgrid_nz ; if (ax % 4 == 0)
- mov dl, ALIVE ; dl = ALIVE
+; setup -----------------------------------------------------------------------
-.initgrid_nz:
- mov [di + cx], dl ; grid[cx] = dl
+setup:
- loop .initgrid ; } while (--cx)
+call init_grid
+call print_grid
jmp halt
; xs() - xorshift pseudorandom number generator -------------------------------
; output:
-; ax = updated state ([XSS])
+; [XSS] = updated state
-; clobbers ax, dx
+; clobbers bx, dx
xs:
-mov ax, [XSS]
-mov dx, ax
+mov bx, [XSS]
+mov dx, bx
shl dx, 1 ; dx = xss << 1
-xor ax, dx ; ax = xss ^ (xss << 1)
-mov dx, ax
+xor bx, dx ; bx = xss ^ (xss << 1)
+mov dx, bx
shr dx, 3 ; dx = xss' >> 3
-xor ax, dx ; ax = xss' ^ (xss' >> 3)
-mov dx, ax
+xor bx, dx ; bx = xss' ^ (xss' >> 3)
+mov dx, bx
shl dx, 10 ; dx = xss'' << 10
-xor ax, dx ; ax = xss'' ^ (xss'' << 10)
-mov [XSS], ax
+xor bx, dx ; bx = xss'' ^ (xss'' << 10)
+mov [XSS], bx
+
+ret
+
+; init_grid() - initialize GRID cells with random values ----------------------
+
+; (*) expects es = 0.
+
+; clobbers ax, bx, cx, dx, di
+
+init_grid:
+
+mov di, GRID ; es:di -> GRID
+mov cx, COLS * ROWS ; for each cell
+
+.write_cell: ; do {
+ call xs ; [XSS] = rand
+ mov al, DEAD ; al = DEAD (likely)
+
+ test [XSS], 0b11
+
+ jnz .nz ; if ([XSS] % 4 == 0)
+ mov al, ALIVE ; al = ALIVE
+
+.nz:
+ stosb ; [es:(di++)] = al
+
+ loop .write_cell ; } while (--cx)
ret
-; errors ----------------------------------------------------------------------
+; print_grid() - print GRID cells to screen -----------------------------------
+
+; (*) this function expects ds = 0 when called and sets es = 0 before
+; returning. this is simpler because in the rest of the program we want the
+; segment registers to be zeroed anyway.
-hello:
-mov si, str.hello
+; clobbers ax, cx, di, si
-; print the error message string in si and halt
-; note: we assume es = VGA_SEG and ds = 0
-printerr:
+print_grid:
+
+mov ax, VGA >> 4
xor di, di
-mov ah, PRINT_COLOR
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+mov es, ax ; es:di -> VGA
+mov si, GRID ; ds:si -> GRID
-write_char:
-lodsb ; al = [ds:si], si += 1
-or al, al ; on null terminator,
-jz halt ; halt
-stosw ; [es:di] = ax, di += 2
-jmp write_char
+mov ah, PRINT_COLOR ; ah = color attribute
+
+mov cx, COLS * ROWS ; for each cell
+
+.print_cell: ; do {
+
+ lodsb ; al = [ds:(si++)]
+ stosw ; [es:di] = ax ; di += 2
+
+ loop .print_cell ; } while (--cx)
+
+mov es, cx ; es = 0
+ret
+
+; halt() - stop program execution ---------------------------------------------
halt:
-cli ; disable interrupts
+
+cli ; disable interrupts
hlt
jmp halt
-; data ------------------------------------------------------------------------
-
-str:
-.hello:
- db "lifeboot", 0
+; -----------------------------------------------------------------------------
times 510 - ($ - $$) db 0 ; fill remaining bytes with zeroes
dw 0xaa55 ; mbr magic byte
projects / lifeboot.git / blobdiff