working box drawing example

test/box.asm.lisp

@@ -1,20 +1,67 @@
 ((code
   (label main
+    ; Open screen
     (load-immediate $0 &screen-namespace)
     (load-immediate $1 0)
     (syscall DEVICE-OPEN $0 $1)
 
-    (load-immediate $3 0xE0)
-    (load-immediate $4 36)
+    ; Draw red box at (10, 20)
+    (load-immediate $1 0xE0)   ; color
+    (push $1)
+    (load-immediate $1 10)     ; x
+    (push $1)
+    (load-immediate $1 20)     ; y
+    (push $1)
+    (call &draw-box)
 
-	(load-immediate $1 1)
-	(load-immediate $5 50)
-	(label loop-x
-		(store-8 $4 $3)
-		(add-int $4 $4 $1)
-		(jump-lt-int &loop-x $4 $5))
+    ; Draw green box at (10, 40)
+    (load-immediate $1 0x1C)   ; RGB332 green (00011100)
+    (push $1)
+    (load-immediate $1 10)
+    (push $1)
+    (load-immediate $1 40)
+    (push $1)
+    (call &draw-box)
 
+    ; Flush and halt
     (syscall DEVICE-WRITE $0 $0 $0)
-    (halt)))
+    (halt))
+
+  ; draw-box(color, x, y)
+  ; Pops: y, x, color
+  (label draw-box
+    ; Pop arguments (reverse order)
+    (pop $13)        ; y_start
+    (pop $11)        ; x_start
+    (pop $3)         ; color
+
+    ; Constants
+    (load-immediate $1 1)        ; increment
+    (load-immediate $2 640)      ; row stride
+    (load-immediate $10 36)      ; framebuffer base
+    (load-immediate $12 20)      ; width
+    (load-immediate $14 20)      ; height
+
+    ; Compute start address: base + y*640 + x
+    (mul-int $15 $13 $2)         ; $15 = y * 640
+    (add-int $15 $15 $11)        ; $15 += x
+    (add-int $4 $10 $15)         ; $4 = base + offset
+
+    ; Outer loop: height times
+    (register-move $5 $14)       ; $5 = row counter
+
+    (label draw-box-outer
+      (add-int $6 $4 $12)        ; $6 = row end = current + width
+      (register-move $7 $4)      ; $7 = pixel pointer
+
+      (label draw-box-inner
+        (store-8 $7 $3)          ; write color
+        (add-int $7 $7 $1)       ; next pixel
+        (jump-lt-int &draw-box-inner $7 $6))
+
+      (add-int $4 $4 $2)         ; next row (+= 640)
+      (sub-int $5 $5 $1)         ; decrement row count
+      (jump-gt-int &draw-box-outer $5 0))
+    (return)))
 (data
   (label screen-namespace "/dev/screen/0")))