add delay() and use farptrs to load segment registers

[lifeboot.git] / lifeboot.asm

diff --git a/lifeboot.asm b/lifeboot.asm
index 81185ddd29816e688d1c8ec713df5a4be2cd1fae..f1545209a35feff333e53ab7ab1eadf0af8b45bf 100644 (file)
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -9,8 +9,8 @@ org 0x7c00
 
 %define DEAD ' '            ; char to represent a dead cell
 %define ALIVE '#'           ; char to represent an alive cell
-%define INIT_RATIO 4        ; dead / alive
 %define PRINT_COLOR 0x07    ; grey on black
+%define WAIT_DELAY 0x02     ; 0.131072 seconds
 
 %define GRID        ORG + 512               ; current cell grid (80 * 25 bytes)
 %define NEXT_GRID   GRID + COLS * ROWS      ; next cell grid (80 * 25 bytes)
@@ -19,99 +19,149 @@ org 0x7c00
 ; entry point -----------------------------------------------------------------
 
 ; initialize segment registers
-xor ax, ax
-mov ds, ax
-mov es, ax
-mov ss, ax
+lds ax, [farptr.zero]
+les ax, [farptr.zero]
+lss ax, [farptr.zero]
 
 ; 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
 
-; 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
+; initialize xss
+mov ah, 0x00                ; get
+int 0x1a                    ;  system time
+mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
 
 ; setup -----------------------------------------------------------------------
 
 setup:
 
-; initialize xss
-mov ah, 0x00                ; get
-int 0x1a                    ;  system time
-mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
+call init_grid
+call print_grid
+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
+
 ; xs() - xorshift pseudorandom number generator -------------------------------
 
 ; output:
-; ax        = updated state ([XSS])
+; [XSS]     = updated state
+
+; 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 al, 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 word [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 -----------------------------------
 
-hello:
-mov si, str.hello
+; (*) 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.
 
-; 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
+; clobbers ax, cx, di, si
 
-; es:di = video memory
-; ds:si = error message
-; al = current char
-; ah = color attribute
+print_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
+les di, [farptr.vga_start]  ; es:di -> VGA
+mov si, GRID                ; ds:si -> GRID
+
+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
+farptr:                     ; offset, segment
+.zero:
+    dw 0                    ; take advantage of next dw 0
+
+.vga_start:
+    dw 0, VGA >> 4
 
 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 dw 0xaa55                   ; mbr magic byte