Start debugging arithmetic

Author: commonkestrel

Arch.md

@@ -39,7 +39,7 @@ ALU operations based on these opcodes are shown below:
 |  `1`  |  `0`  |  `0`  |  `0`  |   `ADD`   |
 |  `1`  |  `0`  |  `0`  |  `1`  |   `SUB`   |
 |  `1`  |  `0`  |  `1`  |  `0`  |   `ADC`   |
-|  `1`  |  `0`  |  `1`  |  `0`  |   `SBB`   |
+|  `1`  |  `0`  |  `1`  |  `1`  |   `SBB`   |
 |  `1`  |  `1`  |  `0`  |  `0`  |  `NAND`   |
 |  `1`  |  `1`  |  `0`  |  `1`  |    `OR`   |
 

examples/box.asm

@@ -51,6 +51,26 @@ draw_top:
     ret
 
 draw_bottom:
+    mv A, 1 ; A contains the X coordinate
+    push A, (1 + BOX_HEIGHT), BL_CORNER ; x, y, character
+    call [draw_character]
+    mv B, (BOX_WIDTH - 2)
+.loop:
+    inc A
+    push A, B
+
+    push A, (1+BOX_HEIGHT), DASH
+    call [draw_character]
+
+    pop B, A
+
+    dec B
+    jnz B, [.loop]
+
+    inc A
+    push A, (1+BOX_HEIGHT), BR_CORNER
+    call [draw_character]
+
     ret
 
 draw_left:
@@ -85,13 +105,14 @@ draw_character:
     pop H, L ; save return address
     pop C, B, A ; character, Y, X
     push L, H ; store return address
-    push A, C ; save X coordinate and character
+    push A, C; save X coordinate and character
+    jeq B, (1+BOX_HEIGHT), [dbg]
 
     push B, 0, SCREEN_WIDTH, 0
     call [mul16] ; get Y offset
     pop H, L ; get value
 
-    pop C, A ; get character X coordinate
+    pop C, A ; get character and X coordinate
 
     add16 H, L, (TEXT_BUFFER >> 8), (TEXT_BUFFER & 0xFF) ; Shift address to text-buffer space
     add16 H, L, 0, A ; add X to address
@@ -100,5 +121,11 @@ draw_character:
 
     ret
 
+dbg:
+    push B, 0, SCREEN_WIDTH, 0
+    call [mul16] ; get Y offset
+    pop H, L ; get value
+    halt
+
 hello_str:
     @str "Hello world!"