{"id":3205,"date":"2025-10-23T16:33:44","date_gmt":"2025-10-23T15:33:44","guid":{"rendered":"https:\/\/www.robertprice.co.uk\/robblog\/?p=3205"},"modified":"2025-10-23T16:33:45","modified_gmt":"2025-10-23T15:33:45","slug":"rc2025-part-7-moving-a-pixel-with-a-nes-controller","status":"publish","type":"post","link":"https:\/\/www.robertprice.co.uk\/robblog\/rc2025-part-7-moving-a-pixel-with-a-nes-controller\/","title":{"rendered":"RC2025 – Part 7 – Moving a pixel with a NES controller"},"content":{"rendered":"\n

In previous posts, I’ve covered how to read an NES controller from an RC2014 computer, and also how to use an 8×8 LED matrix<\/a>. I now want to combine these and use the NES controller to move a pixel on the LED matrix.<\/p>\n\n\n\n

The first thing we want to do is to draw a pixel in the top left corner of the LED matrix. To do this, I’ll use register b for the Row data and register c for the Column data. We need to set a single bit in each because we only want a single pixel lit.<\/p>\n\n\n\n

        ORG $9000\n\nROW     EQU 0\nCOLUMN  EQU 2\n\nsetup:\n        ld b, 0b00000001  ; Row bit for the pixel\n        ld c, 0b10000000  ; Column bit for the pixel\n\nmain:\n\n.draw_pixel:\n        xor a             ; clear a\n        out (COLUMN), a   ; clear the columns\n        ld a, b           ; get the current row bit\n        out (ROW), a      ; select the row\n        ld a, c           ; get the column data for this row\n        out (COLUMN), a   ; output the column data\n\n        jr main           ; repeat forever\n\n        END<\/code><\/pre>\n\n\n\n
Now that we have a pixel, we need to read the NES controller and react to movement on the joypad.<\/p>\n\n\n\n
We can use Z80 rlc<\/code> and rrc<\/code> operations to move the single bit to the left or right in the Row and Column registers. If we reach the end of the byte, the bit will roll over to the other end of the byte.<\/p>\n\n\n\n
To test which button is being pressed, we can use a bit<\/code> operation. If the button isn’t being pressed, we can jump ahead to test the next button.<\/p>\n\n\n\n
In our main loop, we can add the following.<\/p>\n\n\n\n
UP_BTN   EQU $04\nDOWN_BTN EQU $05\nLEFT_BTN EQU $06\nRIGHT_BTN EQU $07\n\nmain:\n        call get_buttons   ; Get NES controller button states in A\n\n.test_left:\n        bit LEFT_BTN, a\n        jr nz, .test_right\n        rlc c\n.test_right:\n        bit RIGHT_BTN, a\n        jr nz, .test_up\n        rrc c\n.test_up:\n        bit UP_BTN, a\n        jr nz, .test_down\n        rrc b\n.test_down:\n        bit DOWN_BTN, a\n        jr nz, .draw_pixel\n        rlc b\n\n.draw_pixel:<\/code><\/pre>\n\n\n\n
We also need to include the get_buttons routine we previously wrote<\/a>. This returns the buttons being pressed on the NES controller in register a.<\/p>\n\n\n\n
        INCLUDE \"NESController.inc\"\n\n        END<\/code><\/pre>\n\n\n\n
At the moment, this is executing far too fast, so we end up with an entire row or column being lit. We need to add a delay. We can write a very simple Z80 routine to just loop X times as a simple delay.<\/p>\n\n\n\n
delay:\n        ld de, 50000\n.delay_loop:\n        dec de\n        ld a, d\n        or e\n        jr nz, .delay_loop\n        ret<\/code><\/pre>\n\n\n\n
The final working code<\/h2>\n\n\n\n
Putting this all together, we end up with the following code…<\/p>\n\n\n\n
\n; A simple program to move a pixel on an 8x8 LED matrix\n; connected to the RC2014 Z80 computer.\n; Robert Price - 19th October 2025\n\n        ORG $9000\n\nROW     EQU 0\nCOLUMN  EQU 2\n\nUP_BTN   EQU $04\nDOWN_BTN EQU $05\nLEFT_BTN EQU $06\nRIGHT_BTN EQU $07\n\nsetup:\n        ld b, 0b00000001  ; Row bit for the pixel\n        ld c, 0b10000000  ; Column bit for the pixel\n\nmain:\n        call get_buttons   ; Get NES controller button states in A\n\n.test_left:\n        bit LEFT_BTN, a\n        jr nz, .test_right\n        rlc c\n.test_right:\n        bit RIGHT_BTN, a\n        jr nz, .test_up\n        rrc c\n.test_up:\n        bit UP_BTN, a\n        jr nz, .test_down\n        rrc b\n.test_down:\n        bit DOWN_BTN, a\n        jr nz, .draw_pixel\n        rlc b\n\n.draw_pixel:\n        xor a             ; clear a\n        out (COLUMN), a   ; clear the columns\n        ld a, b           ; get the current row bit\n        out (ROW), a      ; select the row\n        ld a, c           ; get the column data for this row\n        out (COLUMN), a   ; output the column data\n\n        call delay        ; small delay to make movement visible\n\n        jr main           ; repeat forever\n\ndelay:\n        ld de, 50000\n.delay_loop:\n        dec de\n        ld a, d\n        or e\n        jr nz, .delay_loop\n        ret\n\n        INCLUDE \"NESController.inc\"\n\n        END<\/code><\/pre>\n\n\n\n
Here is a video of the code in action.<\/p>\n\n\n\n
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