Robert Price

So far in this year’s Retro Challenge I’ve designed and built my own Rotary Encoder Module for the RC2014 computer. I’ve also worked out how to control an LCD screen from Z80 assembly language. I now want to combine the two and use the rotary encoder to scroll text on the LCD screen.

I’m building this on the RC2014 Classic 2, so I don’t have access to a file system. I will have to hardcode the text into the program.

I’ve chosen to use the classic hacker song, Puff The Fractal Dragon.

The LCD screen is 20 characters wide, so I will make things easy for myself and ensure every line is 20 characters long. I will pad shorter lines with spaces if necessary.

I’m going to need a pointer to store my current position in the text. I’m calling this puffpointer. I also need to know the start of the text, I’m calling this puff. I’ll also need to know 4 lines before the end of the text. I’m calling this maxpuff. This is calculated in the assembler as the end of the text – 80. The 80 is 4 lines * 20 characters.

I’m using the right turn to scroll down the text, and the left turn to scroll back to the top.

In the right turn I need see if I’m at the end of the text or not. I need to compare puffpointer to maxpuff to see if they match. If they do, I’m at the button so I don’t want to go any further.

The Z80 doesn’t allow us to directly compare 16bit values, so we have to do a bit of a workaround. We can instead clear the a register, then load the values we want to compare into de and hl register pairs. We can then subtract de from hl, and add de back to hl. If they are the same value the Z flag will be set so can test this. In this case, if Z is set we don’t want to do anything else so we can jump back to the main program loop.

    or a
    ld de,maxpuff
    ld hl,(puffpointer)
    sbc hl, de
    add hl, de
    jp z,loop

So if we are get past this point, we are safe to scroll down. We load the pointer to the current line in the text and add 20 to it. This moves us down a line. We then save it, and call our display routine.

    ld hl,(puffpointer)
    ld bc,20
    add hl,bc
    ld (puffpointer),hl
    call show_four_lines

When turning left do a very similar procedure, except we check if puffpointer is at the start of the text. If it isn’t we subtract 20 from puffpointer.

Our final code looks like this.

    OUTPUT LCDScroll.z80

    ORG $9000

ROTARYENCODER EQU $DE
LCD_R   EQU 218
LCD_D   EQU 219

; The input bits from the rotary encoder.
CLK1    EQU %00000001
DT1     EQU %00000010
SW1     EQU %00000100

; show the inital first 4 lines on the LCD.
    call setup_LCD

    ld hl,(puffpointer)      ; the address of the text
    call show_four_lines

loop:
; load the last clk value into register b
    ld  a,(lastclk)
    ld  b,a

; read the input port and store in "input"
    in  a,(ROTARYENCODER)
    ld  (input),a

; now check if the switch on first rotary encoder has been
; pressed. If it has jump to end
    and SW1
    cp  SW1
    jr  z, end

; now see if clk1 matches the lastclk. If it does loop
    ld  a,(input)
    and CLK1
    ld  (lastclk),a
    cp  b
    jr  z, loop

; now work out what direction we are moving.
; if CLK1 is 1 then we can can check DT1 to get the
; direction of rotation. If it's 0, we need to go 
; back to the start of the loop.
    ld  a,(input)          
    and CLK1
    cp  CLK1 
    jr  nz, loop            

; this is where we check DT1. If 1 we are turning left.
    ld  a, (input)
    and DT1
    cp  0 
    jr  nz, left

; we must be turning right, so we need to advance 
; our text. We see if we are at the maximum, and
; if not we advance a line and display.
right:
    or a
    ld de,maxpuff
    ld hl,(puffpointer)
    sbc hl, de
    add hl, de
    jp z,loop

    ld hl,(puffpointer)
    ld bc,20
    add hl,bc
    ld (puffpointer),hl
    call show_four_lines

    jr  loop

; we must be turning left, so we need to go
; back. We see if we are at the start of the 
; text and if not we go back a line and display.
left:
    or a
    ld de,puff
    ld hl,(puffpointer)
    sbc hl, de
    add hl, de
    jp z,loop

    ld hl,(puffpointer)
    ld bc,20
    sub hl,bc
    ld (puffpointer),hl
    call show_four_lines

    jp  loop

; the switch has been pressed, so we clear the output
; and exit.
end:
    call clear_screen

    ret


; Sends a command byte to the LCD.
; A - Command in
; A, C registers used.
send_command:
    out (LCD_R),a
.lcd_busy:
    in a,(LCD_R)
    rlca
    jr c,.lcd_busy
    ret

; Sends a data byte to the LCD
; A - Byte in
; A, C registers used.
send_data:
    out (LCD_D),a
.lcd_busy:
    in a,(LCD_R)
    rlca
    jr c,.lcd_busy
    ret

; setup the LCD screen
setup_LCD:
    ld a,56         ; Function 8 bit, 2 lines, 5x8 dot font
    call send_command
    ld a,12         ; Display on, cursor off, no blink
    call send_command

    call clear_screen

    ret

; clear the LCD screen
clear_screen:
    ld a,1          ; clear the display
    call send_command
    ret

; Display 4 lines of consecutive text on the LCD
; lines are shown 1-20,41-60,21-40,61-80 so we 
; need to jump around to display in order.
; HL - address of text to display on the LCD
; A, B, C, D, E, H, L registers used.
show_four_lines:

; show the first 20 lines
    ld b,20
.line1loop:
    ld a,(hl)
    inc hl
    call send_data
    djnz .line1loop

; jump forward 20 characters, and show
    ld de,20
    add hl,de
    ld b,20
.line2loop:
    ld a,(hl)
    inc hl
    call send_data
    djnz .line2loop    

; jump back 40 characters, and show
    ld de,40
    sub hl,de
    ld b,20
.line3loop:
    ld a,(hl)
    inc hl
    call send_data
    djnz .line3loop

; jump forward 20 characters, and show
    ld de,20
    add hl,de
    ld b,20
.line4loop:
    ld a,(hl)
    inc hl
    call send_data
    djnz .line4loop 

    ret      


; stores the current input from the rotary encode.
input:
    db  0
; stores the last value of CLK1.
lastclk:
    db  0

; stores a pointer to our current position in the text.
puffpointer:
    dw  puff
; the text to show, each line must be 20 bytes long.
puff:
    db "Puff the fractal    "
    db "dragon was written  "
    db "in C,               "
    db "And frolicked while "
    db "processes switched  "
    db "in mainframe memory."
    db "                    "
    db "No plain fanfold    "
    db "paper could hold    "
    db "that fractal Puff   "
    db "                    "
    db "He grew so fast no  "
    db "plotting pack could "
    db "shrink him far      "
    db "enough.             "
    db "Compiles and        "
    db "simulations grew so "
    db "quickly tame        "
    db "And swapped out all "
    db "their data space    "
    db "when Puff pushed    "
    db "his stack frame.    "
    db "                    "
    db "Puff the fractal    "
    db "dragon was written  "
    db "in C,               "
    db "And frolicked while "
    db "processes switched  "
    db "in mainframe memory."
    db "Puff the fractal    "
    db "dragon was written  "
    db "in C,               "
    db "And frolicked while "
    db "processes switched  "
    db "in mainframe memory."
    db "                    "
    db "Puff, he grew so    "
    db "quickly, while      "
    db "others moved like   "
    db "snails              "
    db "And mini-Puffs      "
    db "would perch         "
    db "themselves on his   "
    db "gigantic tail.      "
    db "All the student     "
    db "hackers loved that  "
    db "fractal Puff        "
    db "But DCS did not     "
    db "like Puff, and      "
    db "finally said,       "
    db "\"Enough!\"           "
    db "                    "
    db "Puff the fractal    "
    db "dragon was written  "
    db "in C,               "
    db "And frolicked while "
    db "processes switched  "
    db "in mainframe memory."
    db "Puff the fractal    "
    db "dragon was written  "
    db "in C,               "
    db "And frolicked while "
    db "processes switched  "
    db "in mainframe memory."
    db "                    "
    db "Puff used more      "
    db "resources than DCS  "
    db "could spare.        "
    db "The operator killed "
    db "Puff's job -- he    "
    db "didn't seem to care."
    db "A gloom fell on the "
    db "hackers; it seemed  "
    db "to be the end,      "
    db "But Puff trapped    "
    db "the exception, and  "
    db "grew from naught    "
    db "again!              "
    db "                    "
    db "Puff the fractal    "
    db "dragon was written  "
    db "in C,               "
    db "And frolicked while "
    db "processes switched  "
    db "in mainframe memory."
    db "Puff the fractal    "
    db "dragon was written  "
    db "in C,               "
    db "And frolicked while "
    db "processes switched  "
    db "in mainframe memory."
puffend:
maxpuff EQU puffend - 80

Here’s a video of the rotary encoder in action scrolling through the text of Puff The Fractal Dragon.