Ben Eater 6502 Kit — Day 14 – Add display and print “Hello, World!”

The computer is coming along nicely. Next a Display is added to the system.

The display hangs a bit over the side of the board as the SN74LS21 takes up some extra space. But already I only need only 2kb of address space instead of 8kb like the Ben Eater original.

Thanks to the macro assembler the source code is a lot shorter then the version from Ben Eater. One could probably shorten it further but I’m into over engineering.

.listbytes  16
.pagelength 66
.case     +

;;
; VIA register
;
.segment    "VIA"
.scope     VIA
IORB     =  $7F00   ; Output Register "B" Input Register "B"
IORA     =  $7F01   ; Output Register "A" Input Register "A"
DDRB     =  $7F02   ; Data Direction Register "B"
DDRA     =  $7F03   ; Data Direction Register "A"
T1C_L     =  $7F04   ; T1 Low-Order
T1C_H     =  $7F05   ; T1 High-Order Counter
T1L_L     =  $7F06   ; T1 Low-Order Latches
T1L_H     =  $7F07   ; T1 High-Order Latches
T2C_L     =  $7F08   ; T2 Low-Order
T2C_H     =  $7F09   ; T2 High-Order Counter
SR     =  $7F0A   ; Shift Register
ACR     =  $7F0B   ; Auxiliary Control Register
PCR     =  $7F0C   ; Peripheral Control Register
IFR     =  $7F0D   ; Interrupt Flag Register
IER     =  $7F0E   ; Interrupt Enable Register
RA     =  $7F0F   ; Same as Reg 1 except no "Handshake"

;;
;   Set VIA data direction register A
;
.macro     Set_A Value
    LDA  Value
    STA  VIA::DDRA
.endmacro

;;
;   Set VIA data direction register B
;
.macro     Set_B Value
    LDA  Value
    STA  VIA::DDRB
.endmacro

;;
;   Set VIA output register A
;
.macro     Out_A Value
    LDA  Value
    STA  VIA::IORA
.endmacro

;;
;   Set VIA output register B
;
.macro     Out_B Value
    LDA  Value
    STA  VIA::IORB
.endmacro

.endscope

;;
; LCD Display
;
.scope LCD
E     =  %10000000
RW     =  %01000000
RS     =  %00100000

;;
;   Send Data to LCD display
.macro     Control_LCD Instruction
    Out_B Instruction
    Out_A #0   ; Clear RS, RW, E bits
    Out_A #LCD::E   ; Set E bit to send instruction
    Out_A #0   ; Clear E bit
.endmacro

;;
;   Send Data to LCD display
.macro     Data_LCD Data
    Out_B Data
    Out_A #LCD::RS  ; Set RS, Clear RW, E bits
    Out_A #(LCD::RS | LCD::E) ; Set E bit to send data
    Out_A #LCD::RS  ; Clear E bit
.endmacro

.endscope

.segment    "CODE"

Do_RES:     Set_A #%11100000  ; Set top 3 pin as output
    Set_B #%11111111  ; Set all pins as output

    Control_LCD #%00111000  ; Set 8-bit mode; 2 line display; 5×8 font
    Control_LCD #%00001110  ; Display in; cursor on; blink off
    Control_LCD #%00000110  ; Increment and shift cursor; don't shift display
    Control_LCD #%00000001  ; Clear Display

    Data_LCD #'H'   ; Send character 'H' to display
    Data_LCD #'e'   ; Send character 'e' to display
    Data_LCD #'l'   ; Send character 'l' to display
    Data_LCD #'l'   ; Send character 'l' to display
    Data_LCD #'o'   ; Send character 'o' to display
    Data_LCD #','   ; Send character ',' to display
    Data_LCD #' '   ; Send character ' ' to display
    Data_LCD #'W'   ; Send character 'W' to display
    Data_LCD #'o'   ; Send character 'o' to display
    Data_LCD #'r'   ; Send character 'r' to display
    Data_LCD #'l'   ; Send character 'l' to display
    Data_LCD #'d'   ; Send character 'd' to display
    Data_LCD #'!'   ; Send character '!' to display

    STP     ; Stop Processor

Do_NMI:     RTI

Do_IRQ:     RTI

.segment    "HEADER"
.word     Do_NMI
.word     Do_RES
.word     Do_IRQ

You find the source code for the program with makefile, linker configuration file and assembler source code on GitLab: 6502Tutorial — Tools/Create_LED