modularize init_grid

[lifeboot.git] / lifeboot.asm

diff --git a/lifeboot.asm b/lifeboot.asm
index a3b4658fbbf6f6c1839718c149188cc457a8587a..278bf328a456e12193f8622867feb640975b5ab5 100644 (file)
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -1,16 +1,19 @@
 bits 16
 org 0x7c00
 
 bits 16
 org 0x7c00
 

-%define ORG 0x7c00                  ; where we are loaded initially
+%define ORG 0x7c00
+%define VGA 0xb8000

 
 

-%define VGA_SEG 0xb800              ; video memory starts at 0xb8000
-%define VGA_COL 80
-%define VGA_ROW 25
-%define VGA_LENW VGA_COL * VGA_ROW
+%define COLS 80
+%define ROWS 25

 
 

-%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 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

 
 ; entry point -----------------------------------------------------------------
 
 
 ; entry point -----------------------------------------------------------------
 


@@ -24,55 +27,122 @@ mov ss, ax
 mov sp, ORG
 mov bp, ORG
 
 mov sp, ORG
 mov bp, ORG
 

-; clear screen ----------------------------------------------------------------
+; clear direction flag
+cld

 
 ; disable cursor
 mov ch, 0x3f
 mov ah, 0x01
 int 0x10
 
 
 ; disable cursor
 mov ch, 0x3f
 mov ah, 0x01
 int 0x10
 

-; 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
+
+; setup -----------------------------------------------------------------------
+
+setup:
+
+call init_grid
+call print_grid
+
+jmp halt
+
+; functions -------------------------------------------------------------------
+
+; xs() - xorshift pseudorandom number generator -------------------------------
+
+; 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 with random values ----------------------

 
 

-rep stosw                   ; fill cx words at es:di with ax
+; (*) expects es = 0.

 
 

-; errors ----------------------------------------------------------------------
+; clobbers ax, bx, cx, dx, di

 
 

-hello:
-mov si, str.hello
+init_grid:

 
 

-; print the error message string in si and halt
-; note: we assume es = VGA_SEG and ds = 0
-printerr:
+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
+
+; 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.
+
+; clobbers ax, cx, di, si
+
+print_grid:
+
+mov ax, VGA >> 4

 xor di, di
 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
+
+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

 
 

-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
+    loop .print_cell        ; } while (--cx)
+
+mov es, cx                  ; es = 0
+ret
+
+; halt() - stop program execution ---------------------------------------------

 
 halt:
 
 halt:

-cli                     ; disable interrupts
+
+cli                         ; disable interrupts

 hlt
 jmp halt
 
 hlt
 jmp halt
 

-; data ------------------------------------------------------------------------
-
-str:
-.hello:
-    db "lifeboot", 0
+; -----------------------------------------------------------------------------

 
 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 dw 0xaa55                   ; mbr magic byte
 
 times 510 - ($ - $$) db 0   ; fill remaining bytes with zeroes
 dw 0xaa55                   ; mbr magic byte