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Improving sample program
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Tony Di Nucci
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sample/a.o65
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sample/a.o65
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185
sample/draw.s
185
sample/draw.s
@ -1,33 +1,56 @@
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; simple program that paints cycling colours to the screen using the
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; keyboard curor keys
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; simple program that paints cycling colours to the screen using the keyboard curor keys and will max out your
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; CPU in the process!
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;
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; Esc - ends the process
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;
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; Del - clears the screen - this is HORRIBLY SLOW! There is no clock emulation and so it's not possible within
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; assembly to slow things down, i.e. to prevent painting to screen at lightning speed, meaning you have no real control.
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; In order approximate this a delay of 20ms has been put into the terminal between rendering each pixel change. Since
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; clearing the screen effectively redraws each pixel this means it takes around 64 * 64 * 20ms, so ~82 seconds!!
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; While waiting for this process to complete the program won't respond to you pressing the Esc key.
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;
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; The emulator loads programs at 0x600 so make sure to assemble with text segment at this address.
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;
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; Compile with xa using command:
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; xa draw.s -v -bt 1536
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; draw position
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ldx #$00
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stx $00 ; draw position low
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ldx #$e0
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stx $01 ; draw position high
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; buffer for last key pressed
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key_addr = $f001
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video_bottom_lb = $00
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video_bottom_hb = $e0
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video_top_lb = $00
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video_top_hb = $f0
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video_memory_ptr_lb = $00
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video_memory_ptr_hb = $01
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; address that holds the current colour to draw with
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draw_colour_addr = $10
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draw_colour_addr = $03
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is_clearing_addr = $04
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rubber_colour = $00
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left_key_code = $50
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right_key_code = $4f
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up_key_code = $52
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down_key_code = $51
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esc_key_code = $29
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del_key_code = $4c
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; draw position
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ldx #video_bottom_lb
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stx video_memory_ptr_lb ; starting draw position low
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ldx #video_bottom_hb
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stx video_memory_ptr_hb ; starting draw position high
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; main program loop
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loop
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jsr read_key
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; if we didn't read a key loop back round
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cmp #$0
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cmp #0
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beq loop
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cmp #left_key_code
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@ -49,40 +72,51 @@ loop
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not_up
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cmp #down_key_code
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bne loop
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bne not_down
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jsr move_down
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jmp loop
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not_down
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cmp #del_key_code
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bne not_del
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jsr clear_screen
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jmp loop
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not_del
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cmp #esc_key_code
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bne loop
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jmp exit
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jmp loop
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read_key
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lda key_addr
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; clear the key press buffer so we don't read same key press twice
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ldx #$0
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ldx #0
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stx key_addr
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rts
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move_right
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lda $00
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lda video_memory_ptr_lb
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; if we're at $ff then we need to increment the high byte before rendering
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cmp #$ff
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bne do_right_render
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; if we're at $efff then we have to stop because we'd start writing outside of video memory
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lda $01
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cmp $ef
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lda video_memory_ptr_hb
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cmp #$ef
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bcs after_right_render
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; increment high byte and reset the low one
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inc $01
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ldx #$00
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stx $00
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inc video_memory_ptr_hb
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ldx #0
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stx video_memory_ptr_lb
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do_right_render
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inc $00
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inc video_memory_ptr_lb
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jsr render_dot
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after_right_render
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@ -90,21 +124,21 @@ move_right
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; comments for "move_right" also apply here but direction is obviously reversed
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move_left
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lda $00
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lda video_memory_ptr_lb
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cmp #$00
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cmp #0
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bne do_left_render
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lda $01
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cmp #$e0
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lda video_memory_ptr_hb
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cmp #video_bottom_hb
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beq after_left_render
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dec $01
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ldx #$00
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stx $00
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dec video_memory_ptr_hb
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ldx #0
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stx video_memory_ptr_lb
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do_left_render
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dec $00
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dec video_memory_ptr_lb
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jsr render_dot
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after_left_render
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@ -112,24 +146,24 @@ move_left
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move_up
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; don't want to risk moving into non graphics memory so perform boundary checks
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ldx $00
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ldx video_memory_ptr_lb
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; each line contains $40 (64) pixels, if low byte is above this then we're safe to move
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cpx #$40
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bcs after_up_boundary_check
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; couldn't tell if we're safe by looking at low byte, does the high byte allow move?
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ldx $01
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ldx video_memory_ptr_hb
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cpx #$e0
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beq after_up_render
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after_up_boundary_check
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lda $00
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lda video_memory_ptr_lb
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sbc #$40
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sta $00
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sta video_memory_ptr_lb
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; if carry flag is set then we'll also have to decrement the high byte
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bcs after_up_subtract
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dec $01
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dec video_memory_ptr_hb
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after_up_subtract
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jsr render_dot
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@ -139,19 +173,19 @@ move_up
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; comments for "move_up" also apply here but direction is obviously reversed
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move_down
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ldx $00
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ldx video_memory_ptr_lb
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cpx #$9b
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bcc after_down_boundary_check
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ldx $01
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ldx video_memory_ptr_hb
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cpx #$ef
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beq after_down_render
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after_down_boundary_check
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lda $00
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lda video_memory_ptr_lb
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adc #$40
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sta $00
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sta video_memory_ptr_lb
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bcc after_down_add
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inc $01
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inc video_memory_ptr_hb
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after_down_add
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jsr render_dot
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@ -159,13 +193,78 @@ move_down
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after_down_render
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rts
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; renders a pixel on the screen
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render_dot
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; change the drawing colour (256 colours and we'll wrap around when $ff is hit)
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inc draw_colour_addr
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lda draw_colour_addr
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; if we're clearing the screen then load the rubber colour...
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ldx is_clearing_addr
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cpx #0
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beq set_colour
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lda #rubber_colour
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jmp end_set_colour
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; write the "draw colour" to address pointed to across ($01 $00) - little endian
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ldx #$00
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sta ($00, x)
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; ... else cycle the drawing colour
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set_colour
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; change the drawing colour (256 colours and we'll wrap around when $ff is hit)
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inc draw_colour_addr
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lda draw_colour_addr
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end_set_colour
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; write the "draw colour" to address pointed to across (video_memory_ptr_hb video_memory_ptr_lb)
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ldx #video_memory_ptr_lb
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sta (video_memory_ptr_lb, x)
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rts
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; clears the screen
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clear_screen
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; go into "clear screen" mode
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ldx #1
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stx is_clearing_addr
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ldx #video_bottom_lb
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stx video_memory_ptr_lb
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ldx #video_bottom_hb
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stx video_memory_ptr_hb
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clear_pixel
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; use the existing move_right routine to clear the screen from left-to-right/top-to-bottom
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jsr move_right
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; if we're not at the end of a block then just repeat
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lda video_memory_ptr_lb
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cmp #$ff
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bne clear_pixel
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; we're at the of the block...
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lda video_memory_ptr_hb
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; if we're at the end of the screen then break from loop
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cmp #video_top_hb
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beq end_clear_pixel
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; otherwise move onto the next block
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inc video_memory_ptr_hb
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ldx #0
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stx video_memory_ptr_lb
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jmp clear_pixel
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end_clear_pixel
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; come out of "clear screen" mode
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ldx #0
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stx is_clearing_addr
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; move the draw position back to the top left of screen
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ldx #video_bottom_lb
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stx video_memory_ptr_lb
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ldx #video_bottom_hb
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stx video_memory_ptr_hb
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rts
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; jump to here to end the process
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exit
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