X-Git-Url: https://git.wirehaze.ovh/lifeboot.git/blobdiff_plain/b18225d8568d8f36da6109113621a001a627da1f..90ffff61d4b6169c5bd8ef942ddc269d1da4bce7:/lifeboot.asm?ds=sidebyside

diff --git a/lifeboot.asm b/lifeboot.asm
index a3b4658..0e114d0 100644
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -1,78 +1,189 @@
 bits 16
 org 0x7c00
 
-%define ORG 0x7c00                  ; where we are loaded initially
+; =============================================================================
 
-%define VGA_SEG 0xb800              ; video memory starts at 0xb8000
-%define VGA_COL 80
-%define VGA_ROW 25
-%define VGA_LENW VGA_COL * VGA_ROW
+; settings --------------------------------------------------------------------
 
-%define FILL_CHAR 0xfa              ; middle dot
-%define FILL_COLOR 0x04             ; red on black
-%define PRINT_COLOR 0x07            ; grey on black
+%define DEAD ' '            ; char to represent a dead cell
+%define ALIVE '#'           ; char to represent an alive cell
+%define PRINT_COLOR 0x07    ; grey on black
+%define WAIT_DELAY 0x02     ; 0.131072 seconds
+
+; constants -------------------------------------------------------------------
+
+%define COLS 80
+%define ROWS 25
+%define VGAPGSZ (COLS * ROWS * 2)
+
+; memory layout ---------------------------------------------------------------
+
+%define ORG 0x7c00
+%define VGA 0xb8000
+%define DAT ORG + 512
+
+; static variables ------------------------------------------------------------
+
+%define xss DAT             ; xs() state (word)
+%define currvgapg DAT + 2   ; current vga page near pointer (word)
+
+; =============================================================================
 
 ; entry point -----------------------------------------------------------------
 
 ; initialize segment registers
-xor ax, ax
+mov ax, VGA >> 4
 mov ds, ax
 mov es, ax
+
+xor ax, ax
 mov ss, ax
 
 ; 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, 0x3f                ; cursor start and options
+mov ah, 0x01                ; set text-mode cursor shape
+int 0x10                    ; video services
 
-; clear video memory
-mov cx, VGA_LENW
-mov ax, VGA_SEG
-mov es, ax
-xor di, di
-mov ax, (FILL_COLOR << 8) | FILL_CHAR
+; initialize xss
+mov ah, 0x00                ; get
+int 0x1a                    ;  system time
+mov [xss], dx               ; cx:dx = number of clock ticks since midnight
+
+; initialize currvgapg
+mov word [currvgapg], VGAPGSZ
+
+; setup -----------------------------------------------------------------------
+
+setup:
+
+call init_grid
+call flip_vga_page
+call delay
+
+jmp setup
+
+; =============================================================================
+
+; functions -------------------------------------------------------------------
+
+; delay() - suspend program execution temporarily -----------------------------
+
+; clobbers ah, cx, dx
+
+delay:
+
+mov cx, WAIT_DELAY          ; cx:dx = interval in microseconds
+mov dx, 0
+
+mov ah, 0x86
+int 0x15                    ; wait
+
+ret
+
+; vsync_wait() - wait for display to enter the next VBlank cycle --------------
+
+; clobbers ax, dx
+
+vsync_wait:
 
-rep stosw                   ; fill cx words at es:di with ax
+mov dx, 0x3da               ; input status #1 register
 
-; errors ----------------------------------------------------------------------
+.wait_on:
+    in al, dx
+    test al, 0x08           ; vertical retrace bit
+    jnz .wait_on
 
-hello:
-mov si, str.hello
+.wait_off:
+    in al, dx
+    test al, 0x08
+    jz .wait_off
 
-; 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
+ret
 
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+; xs() - xorshift pseudorandom number generator -------------------------------
 
-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
+; output:
+; [xss]     = updated state
+
+; clobbers bx, dx
+
+xs:
+
+mov bx, [xss]
+mov dx, bx
+
+shl dx, 1                   ; dx = xss << 1
+xor bx, dx                  ; bx = xss ^ (xss << 1)
+mov dx, bx
+
+shr dx, 3                   ; dx = xss' >> 3
+xor bx, dx                  ; bx = xss' ^ (xss' >> 3)
+mov dx, bx
+
+shl dx, 10                  ; dx = xss'' << 10
+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                 ;     [xss] = rand
+    mov al, DEAD            ;     al = DEAD (likely)
+
+    test word [xss], 0b11
+
+    jnz  .nz                ;     if ([xss] % 4 == 0)
+    mov al, ALIVE           ;         al = ALIVE
+
+.nz:
+    stosw                   ;     [es:di] = ax ; di += 2
+
+    loop .write_cell        ; } while (--cx)
+
+ret
+
+; flip_vga_page() - flip active display page ----------------------------------
+
+; clobbers ax, dx
+
+flip_vga_page:
+
+call vsync_wait
+
+xor word [currvgapg], VGAPGSZ   ; flip currvgapg
+setnz al                        ; al = !(currvgapg == 0)
+
+mov ah, 0x05                ; select active display page
+int 0x10                    ; video services
+
+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