Improving sample program

sample/draw.s

@@ -1,33 +1,56 @@
-; simple program that paints cycling colours to the screen using the
+; simple program that paints cycling colours to the screen using the keyboard curor keys and will max out your
-; keyboard curor keys
+; CPU in the process!
+;
+; Esc - ends the process
+;
+; Del - clears the screen - this is HORRIBLY SLOW!  There is no clock emulation and so it's not possible within
+; assembly to slow things down, i.e. to prevent painting to screen at lightning speed, meaning you have no real control.
+; In order approximate this a delay of 20ms has been put into the terminal between rendering each pixel change.  Since
+; clearing the screen effectively redraws each pixel this means it takes around 64 * 64 * 20ms, so ~82 seconds!!
+; While waiting for this process to complete the program won't respond to you pressing the Esc key.
 ;
 ; The emulator loads programs at 0x600 so make sure to assemble with text segment at this address.
+;
 ; Compile with xa using command:
 ; xa draw.s -v -bt 1536
 
-; draw position
-ldx #$00
-stx $00             ; draw position low
-ldx #$e0
-stx $01             ; draw position high
-
 ; buffer for last key pressed
 key_addr = $f001
 
+video_bottom_lb = $00
+video_bottom_hb = $e0
+
+video_top_lb = $00
+video_top_hb = $f0
+
+video_memory_ptr_lb = $00
+video_memory_ptr_hb = $01
+
 ; address that holds the current colour to draw with
-draw_colour_addr = $10
+draw_colour_addr = $03
+is_clearing_addr = $04
+
+rubber_colour = $00
 
 left_key_code = $50
 right_key_code = $4f
 up_key_code = $52
 down_key_code = $51
+esc_key_code = $29
+del_key_code = $4c
+
+; draw position
+ldx #video_bottom_lb
+stx video_memory_ptr_lb       ; starting draw position low
+ldx #video_bottom_hb
+stx video_memory_ptr_hb       ; starting draw position high
 
 ; main program loop
 loop
     jsr read_key
 
     ; if we didn't read a key loop back round
-    cmp #$0
+    cmp #0
     beq loop
 
     cmp #left_key_code
@@ -49,40 +72,51 @@ loop
 
             not_up
                 cmp #down_key_code
-                bne loop
+                bne not_down
                 jsr move_down
                 jmp loop
 
+                not_down
+                    cmp #del_key_code
+                    bne not_del
+                    jsr clear_screen
+                    jmp loop
+
+                    not_del
+                        cmp #esc_key_code
+                        bne loop
+                        jmp exit
+
     jmp loop
 
 read_key
     lda key_addr
 
     ; clear the key press buffer so we don't read same key press twice
-    ldx #$0
+    ldx #0
     stx key_addr
 
     rts
 
 move_right
-    lda $00
+    lda video_memory_ptr_lb
 
     ; if we're at $ff then we need to increment the high byte before rendering
     cmp #$ff
     bne do_right_render
 
     ; if we're at $efff then we have to stop because we'd start writing outside of video memory
-    lda $01
+    lda video_memory_ptr_hb
-    cmp $ef
+    cmp #$ef
     bcs after_right_render
 
     ; increment high byte and reset the low one
-    inc $01
+    inc video_memory_ptr_hb
-    ldx #$00
+    ldx #0
-    stx $00
+    stx video_memory_ptr_lb
 
     do_right_render
-        inc $00
+        inc video_memory_ptr_lb
         jsr render_dot
 
         after_right_render
@@ -90,21 +124,21 @@ move_right
 
 ; comments for "move_right" also apply here but direction is obviously reversed
 move_left
-    lda $00
+    lda video_memory_ptr_lb
 
-    cmp #$00
+    cmp #0
     bne do_left_render
 
-    lda $01
+    lda video_memory_ptr_hb
-    cmp #$e0
+    cmp #video_bottom_hb
     beq after_left_render
 
-    dec $01
+    dec video_memory_ptr_hb
-    ldx #$00
+    ldx #0
-    stx $00
+    stx video_memory_ptr_lb
 
     do_left_render
-        dec $00
+        dec video_memory_ptr_lb
         jsr render_dot
 
         after_left_render
@@ -112,24 +146,24 @@ move_left
 
 move_up
     ; don't want to risk moving into non graphics memory so perform boundary checks
-    ldx $00
+    ldx video_memory_ptr_lb
 
     ; each line contains $40 (64) pixels, if low byte is above this then we're safe to move
     cpx #$40
     bcs after_up_boundary_check
 
     ; couldn't tell if we're safe by looking at low byte, does the high byte allow move?
-    ldx $01
+    ldx video_memory_ptr_hb
     cpx #$e0
     beq after_up_render
 
     after_up_boundary_check
-        lda $00
+        lda video_memory_ptr_lb
         sbc #$40
-        sta $00
+        sta video_memory_ptr_lb
         ; if carry flag is set then we'll also have to decrement the high byte
         bcs after_up_subtract
-        dec $01
+        dec video_memory_ptr_hb
 
         after_up_subtract
             jsr render_dot
@@ -139,19 +173,19 @@ move_up
 
 ; comments for "move_up" also apply here but direction is obviously reversed
 move_down
-    ldx $00
+    ldx video_memory_ptr_lb
     cpx #$9b
     bcc after_down_boundary_check
-    ldx $01
+    ldx video_memory_ptr_hb
     cpx #$ef
     beq after_down_render
 
     after_down_boundary_check
-        lda $00
+        lda video_memory_ptr_lb
         adc #$40
-        sta $00
+        sta video_memory_ptr_lb
         bcc after_down_add
-        inc $01
+        inc video_memory_ptr_hb
 
         after_down_add
             jsr render_dot
@@ -159,13 +193,78 @@ move_down
             after_down_render
                 rts
 
+; renders a pixel on the screen
 render_dot
-    ; change the drawing colour (256 colours and we'll wrap around when $ff is hit)
+    ; if we're clearing the screen then load the rubber colour...
-    inc draw_colour_addr
+    ldx is_clearing_addr
-    lda draw_colour_addr
+    cpx #0
+    beq set_colour
+    lda #rubber_colour
+    jmp end_set_colour
 
-    ; write the "draw colour" to address pointed to across ($01 $00) - little endian
+    ; ... else cycle the drawing colour
-    ldx #$00
+    set_colour
-    sta ($00, x)
+        ; change the drawing colour (256 colours and we'll wrap around when $ff is hit)
+        inc draw_colour_addr
+        lda draw_colour_addr
+
+    end_set_colour
+
+    ; write the "draw colour" to address pointed to across (video_memory_ptr_hb video_memory_ptr_lb)
+    ldx #video_memory_ptr_lb
+    sta (video_memory_ptr_lb, x)
 
     rts
+
+; clears the screen
+clear_screen
+    ; go into "clear screen" mode
+    ldx #1
+    stx is_clearing_addr
+
+    ldx #video_bottom_lb
+    stx video_memory_ptr_lb
+
+    ldx #video_bottom_hb
+    stx video_memory_ptr_hb
+
+    clear_pixel
+        ; use the existing move_right routine to clear the screen from left-to-right/top-to-bottom
+        jsr move_right
+
+        ; if we're not at the end of a block then just repeat
+        lda video_memory_ptr_lb
+        cmp #$ff
+        bne clear_pixel
+
+        ; we're at the of the block...
+        lda video_memory_ptr_hb
+
+        ; if we're at the end of the screen then break from loop
+        cmp #video_top_hb
+        beq end_clear_pixel
+
+        ; otherwise move onto the next block
+        inc video_memory_ptr_hb
+        ldx #0
+        stx video_memory_ptr_lb
+
+        jmp clear_pixel
+
+    end_clear_pixel
+
+    ; come out of "clear screen" mode
+    ldx #0
+    stx is_clearing_addr
+
+    ; move the draw position back to the top left of screen
+    ldx #video_bottom_lb
+    stx video_memory_ptr_lb
+
+    ldx #video_bottom_hb
+    stx video_memory_ptr_hb
+
+    rts
+
+; jump to here to end the process
+exit