Command Line POC

A proof of concept for passing parameters to a machine code program from the BASIC prompt, much like a command line tool on a modern OS. The example is a minimal cat: give it a filename and it prints that file from the SD card.

How to run it

Open the folder in VSCode and press F5. This builds src/cat.bmasm to app/CAT and puts it on the SD card with bit.txt and magic.txt. At the BASIC prompt, type:

^CAT" BIT.TXT

The contents of bit.txt are printed. Try ^CAT" MAGIC.TXT to show the other file.

How it works

A line typed at the BASIC prompt is held in the input buffer at $0200. Loading and running a program with the ^ wedge leaves that buffer alone, so the program can read the rest of the line itself.

First it scans the buffer for the " that closes the program name:

    .const startAddress = 0x200 + 3;

    ldx #0
.loop:
    inx
    beq overflow
    lda startAddress, x
    cmp #'"'
    bne -loop

Then it skips any spaces and copies the rest of the line into buffer, up to the null at the end of the line. Its length is the y register, which is pushed for later.

The parameter is then used as the filename:

1. SETLFS and SETNAM set up logical file 3 on device 8 with the filename from buffer, and OPEN opens it.
2. CHKIN makes it the input channel.
3. A loop reads each character with CHRIN and prints it with CHROUT, until READST reports the end of the file.
4. CLRCHN and CLOSE tidy up, and the program returns to BASIC.

.read_loop:
    jsr CHRIN
    jsr CHROUT

    jsr READST
    beq read_loop       ; any status bit (EOF or error) ends the read

As a proof of concept there’s no error handling. A missing file or a parameter longer than 128 bytes stops at a .exception in the debugger.

View the source on GitHub