X-Git-Url: https://git.wirehaze.ovh/lifeboot.git/blobdiff_plain/9ed4c073c5c0b67258262f362dea3c6cfd666e89..refs/heads/main:/lifeboot.asm?ds=sidebyside

diff --git a/lifeboot.asm b/lifeboot.asm
index b0575ae..739df40 100644
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -1,138 +1,344 @@
 bits 16
-org 0x7c00
+org 7c00h
 
-%define ORG 0x7c00
-%define VGA 0xb8000
+; =============================================================================
+
+; settings --------------------------------------------------------------------
+
+%define DEAD ' '            ; char to represent a dead cell
+%define ALIVE '.'           ; char to represent an alive cell
+%define PRINT_COLOR 07h     ; grey on black
+%define ITER_LIM 500        ; reset the simulation after this # iterations
+
+; constants -------------------------------------------------------------------
 
 %define COLS 80
 %define ROWS 25
+%define VGAPGSZ 1000h       ; (bytes)
 
-%define DEAD ' '            ; char to represent a dead cell
-%define ALIVE '#'           ; char to represent an alive cell
-%define PRINT_COLOR 0x07    ; grey on black
+; memory layout ---------------------------------------------------------------
+
+%define ORG 7c00h
+%define VGA 0b8000h
+%define DAT ORG + 512
+
+; static variables ------------------------------------------------------------
 
-%define GRID        ORG + 512               ; current cell grid (80 * 25 bytes)
-%define NEXT_GRID   GRID + COLS * ROWS      ; next cell grid (80 * 25 bytes)
-%define XSS         NEXT_GRID + COLS * ROWS ; xs() state
+%define xss DAT             ; xs() state (word)
+%define currvgapg DAT + 2   ; current vga page near pointer (word)
+%define iter DAT + 4        ; current iteration # (word)
+
+; =============================================================================
 
 ; entry point -----------------------------------------------------------------
 
+entry:
+
 ; initialize segment registers
+mov ax, VGA >> 4
+mov es, ax                  ; es:0 -> video memory
+
 xor ax, ax
-mov ds, ax
-mov es, ax
-mov ss, ax
+mov ds, ax                  ; ds = 0
+mov ss, ax                  ; ss = 0
 
 ; set stack pointers
-mov sp, ORG
 mov bp, ORG
+mov sp, bp
 
-; clear screen ----------------------------------------------------------------
+; clear direction flag
+cld
 
 ; disable cursor
-mov ch, 0x3f
-mov ah, 0x01
-int 0x10
+mov ch, 3fh                 ; cursor start and options
+mov ah, 01h                 ; set text-mode cursor shape
+int 10h                     ; bios video services
 
-; clear video memory
-mov cx, COLS * ROWS
-mov ax, VGA >> 4
-mov es, ax
-xor di, di
-mov ax, (PRINT_COLOR << 8) | DEAD
+; initialize xss
+.set_xss:
+    mov ah, 00h             ; get system time
+    int 1ah                 ; bios time services
+                            ; cx:dx = # clock ticks since midnight
+    or dx, dx
+    jz .set_xss             ; wait for something meaningful
 
-rep stosw                   ; fill cx words at es:di with ax
+mov [xss], dx
 
-; setup -----------------------------------------------------------------------
+; initialize currvgapg
+mov word [currvgapg], 0
 
-setup:
+; start simulation from random state ------------------------------------------
 
-; initialize xss
-mov ah, 0x00                ; get
-int 0x1a                    ;  system time
-mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
+start:
 
-; initialize grid
-mov di, GRID
-mov cx, COLS * ROWS
+call init_grid
+mov word [iter], 0
 
-.initgrid:                  ; do {
-    call xs                 ;     ax = rand
-    mov dl, DEAD            ;     dl = DEAD (likely)
+.next_state:
 
-    test ax, 0b11
+    call vsync_wait
+    call update_grid
 
-    jnz  .initgrid_nz       ;     if (ax % 4 == 0)
-    mov dl, ALIVE           ;         dl = ALIVE
+    call vsync_wait
+    call flip_vgapg
 
-.initgrid_nz:
-    mov [di + cx], dl       ;     grid[cx] = dl
+    inc word [iter]
+    cmp word [iter], ITER_LIM
+    jl .next_state
 
-    loop .initgrid          ; } while (--cx)
+jmp start                   ; reset
 
-jmp halt
+; =============================================================================
 
 ; functions -------------------------------------------------------------------
 
+; vsync_wait() - wait for display to enter the next retrace cycle -------------
+
+; clobbers al, dx
+
+vsync_wait:
+
+mov dx, 3dah                ; input status #1 register
+
+.wait_retrace_end:
+    in al, dx
+    test al, 08h            ; vertical retrace bit
+    jnz .wait_retrace_end
+
+.wait_retrace_start:
+    in al, dx
+    test al, 08h
+    jz .wait_retrace_start
+
+ret
+
 ; xs() - xorshift pseudorandom number generator -------------------------------
 
 ; output:
-; ax        = updated state ([XSS])
+; [xss]     = updated state
+; bx        = [xss]
 
-; 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 randomly --------------------------------
+
+; clobbers ax, bx, cx, dx, di
+
+init_grid:
+
+mov ah, PRINT_COLOR         ; ah = color attribute
+mov di, [currvgapg]         ; es:di -> grid start
+mov cx, COLS * ROWS         ; for each cell
+
+.write_cell:                ; do {
+    call xs                 ;     bx = random value
+    mov al, DEAD            ;     al = DEAD
+
+    test bx, 0b11
+
+    jnz  .nz                ;     if (bx % 4 == 0)
+    mov al, ALIVE           ;         al = ALIVE
+
+.nz:
+    stosw                   ;     [es:di] = ax, di += 2
+
+    loop .write_cell        ; } while (--cx)
+
+ret
+
+; flip_vgapg() - flip active display page -------------------------------------
+
+; output:
+; [currvgapg] ^= VGAPGSZ
+
+; clobbers ax
+
+flip_vgapg:
+
+xor word [currvgapg], VGAPGSZ
+
+setnz al                    ; al = !(currvgapg == 0)
+mov ah, 05h                 ; select active display page
+int 10h                     ; bios video services
+
+ret
+
+; alive_neighbours() - get number of alive adjacent cells ---------------------
+
+; input:
+; es:si     -> current grid
+; cx        -> cell index
+
+; output:
+; ax = # alive neighbours
 
+; clobbers ax, bx, dx
+
+alive_neighbours:
+enter 5, 0
+
+; [bp - 1]: row
+; [bp - 2]: col
+; [bp - 3]: neighbours
+; [bp - 4]: i
+; [bp - 5]: j
+
+mov ax, cx                  ; ax = idx
+mov bl, COLS                ; bl = COLS
+div bl                      ; al = idx / COLS, ah = idx % COLS
+
+mov [bp - 1], al            ; row = idx / COLS
+mov [bp - 2], ah            ; row = idx % COLS
+mov byte [bp - 3], 0        ; neighbours = 0
+
+mov byte [bp - 4], -1       ; i = -1
+.i:                         ; do {
+
+    mov byte [bp - 5], -1           ; j = -1
+.j:                                 ; do {
+
+        mov al, [bp - 4]
+        or al, [bp - 5]                 ; if (!i && !j)
+        jz .continue                    ;     continue
+
+        mov al, [bp - 1]
+        add al, [bp - 4]                ; al = row + i
+
+        cmp al, ROWS                    ; if ((unsigned)(row + i) >= ROWS)
+        jae .continue                   ;     continue
+
+        mov ah, [bp - 2]
+        add ah, [bp - 5]                ; ah = col + j
+
+        cmp ah, COLS                    ; if ((unsigned)(col + j) >= COLS)
+        jae .continue                   ;     continue
+
+        movzx bx, ah                    ; bx = col + j
+
+        mov ah, COLS
+        mul ah                          ; ax = (row + i) * COLS
+
+        add bx, ax                      ; bx = cell index
+        shl bx, 1                       ; bx = grid offset
+
+        mov dx, [es:si + bx]            ; dl = cell state
+
+        cmp dl, ALIVE                   ; if (!ALIVE)
+        jne .continue                   ;     continue
+
+        inc byte [bp - 3]               ; neighbours++
+
+.continue:
+        inc byte [bp - 5]
+        cmp byte [bp - 5], 1
+        jle .j                      ; } while (++j <= 1)
+
+    inc byte [bp - 4]
+    cmp byte [bp - 4], 1
+    jle .i                  ; } while (++i <= 1)
+
+movzx ax, byte [bp - 3]     ; ax = # alive neighbours
+
+leave
 ret
 
-; errors ----------------------------------------------------------------------
+; update_cell() - write next cell state ---------------------------------------
+
+; input:
+; es:si     -> current grid
+; es:di     -> next grid
+; cx        -> cell index
+
+; output:
+; [es:di + cx * 2] = updated cell state
+
+; clobbers ax, bx, dx
 
-hello:
-mov si, str.hello
+update_cell:
 
-; print the error message string in si and halt
-; note: we assume es = VGA_SEG and ds = 0
-printerr:
-xor di, di
-mov ah, PRINT_COLOR
+call alive_neighbours       ; ax = # alive neighbours
 
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+mov bx, cx
+shl bx, 1
 
-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 dx, [es:si + bx]        ; dl = cell current state
+
+cmp dl, ALIVE               ; if (ALIVE) {
+jne .else
+
+shr ax, 1
+cmp ax, 1                   ;     if (n != 2 || n != 3)
+jne .dead                   ;         return DEAD
+
+jmp .alive                  ;     return ALIVE
+
+.else:                      ; }
+
+cmp ax, 3                   ; if (n != 3)
+jne .dead                   ;     return DEAD
+
+.alive:                     ; return ALIVE
+mov dl, ALIVE
+jmp .write
+
+.dead:
+mov dl, DEAD
+
+.write:
+mov [es:di + bx], dx
+
+ret
+
+; update_grid() - write next grid (inactive vga page) -------------------------
+
+; clobbers di, si, ax, bx, cx, dx
+
+update_grid:
+
+mov si, [currvgapg]         ; es:si -> current page
+
+mov di, VGAPGSZ
+xor di, si                  ; es:di -> next page
+
+mov cx, COLS * ROWS - 1     ; for each cell (index)
+
+.update_loop:               ; do {
+
+    call update_cell
+
+    dec cx
+    jns .update_loop        ; } while (--cx > 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
+dw 0aa55h                   ; mbr magic byte