Ben Eater 6502 Kit — Day 17 – Using subroutines to make the Hello World even more compact!

Now we have some RAM we have stack and can use subroutines to make the program more compact. I already used macros and include files so I can just rewrite the macros and both programs, the one with repeated calls and the one with a loop, will benefit from just one change:

.scope      LCD

E           =           %10000000
RW          =           %01000000
RS          =           %00100000

.pushseg
.segment    "CODE"

;;
;   Send Data to LCD display
;
.proc     Control
    VIA_Out_B
    VIA_Out_A #0   ; Clear RS, RW, E bits
    VIA_Out_A #LCD::E   ; Set E bit to send instruction
    VIA_Out_A #0   ; Clear E bit
    RTS
.endproc

.macro     LCD_Control Instruction
    LDA  Instruction
    JSR  LCD::Control
.endmacro

;;
;   Send Data to LCD display
;
.proc     Data
    VIA_Out_B
    VIA_Out_A #LCD::RS  ; Set RS, Clear RW, E bits
    VIA_Out_A #(LCD::RS | LCD::E) ; Set E bit to send data
    VIA_Out_A #LCD::RS  ; Clear E bit
    RTS
.endproc

.macro     LCD_Data Character
    LDA  Character
    JSR  LCD::Data
.endmacro

.popseg
.endscope

Note how I keep the macros compatible so there are no changes needed to the actual programs. I also use some of the more advanced features of the CA65 assembler like procedures (.proc), modules (.scope) and segmented relocatable code (.segment). The result:

If you wonder about the knife on many of the pictures: It’s the SOG kilowatt. A wire stripping knife for electricians. The best I ever had. I also use it the gently pry the ROM gently from the breadboard.

But no matter how gentle I pry the ROM, it’s already in a very sad state. To anyone doing the project i would suggest to get a high quality DIL socket to protect the ROM. have ordered one, as well as a few spare EEPROMs.

You find the source code for the macros on GitLab: 6502Tutorial — Kit/Library