X-Git-Url: https://git.wirehaze.ovh/lifeboot.git/blobdiff_plain/9ed4c073c5c0b67258262f362dea3c6cfd666e89..9213edc199c093cde09fd5fa4c642017ca1c6501:/lifeboot.asm?ds=sidebyside

diff --git a/lifeboot.asm b/lifeboot.asm
index b0575ae..07d9c5a 100644
--- a/lifeboot.asm
+++ b/lifeboot.asm
@@ -27,22 +27,14 @@ mov ss, ax
 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:
@@ -53,23 +45,25 @@ int 0x1a                    ;  system time
 mov [XSS], dx               ; cx:dx = number of clock ticks since midnight
 
 ; initialize grid
-mov di, GRID
-mov cx, COLS * ROWS
+mov di, GRID                ; es:di -> GRID
+mov cx, COLS * ROWS         ; for each cell
 
 .initgrid:                  ; do {
-    call xs                 ;     ax = rand
-    mov dl, DEAD            ;     dl = DEAD (likely)
+    call xs                 ;     [XSS] = rand
+    mov al, DEAD            ;     al = DEAD (likely)
 
-    test ax, 0b11
+    test [XSS], 0b11
 
-    jnz  .initgrid_nz       ;     if (ax % 4 == 0)
-    mov dl, ALIVE           ;         dl = ALIVE
+    jnz  .initgrid_nz       ;     if ([XSS] % 4 == 0)
+    mov al, ALIVE           ;         al = ALIVE
 
 .initgrid_nz:
-    mov [di + cx], dl       ;     grid[cx] = dl
+    stosb                   ;     [es:(di++)] = al
 
     loop .initgrid          ; } while (--cx)
 
+call print_grid
+
 jmp halt
 
 ; functions -------------------------------------------------------------------
@@ -77,62 +71,68 @@ 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
 
-; 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