Sfotty Pie CLI

Sfotty Pie CLI is an emulator for a virtual 6502-based computer. The current implementation allows you two write CLI programs with access to stdin, stdout, and stderr.

Executable file format

| Offset | Size | Description |
| ------ | -------- | ----------------------------------------- |
| 0x0000 | 0x06 | Magic number (The word SFOTTY in ASCII) |
| 0x0006 | 0x04 | Reserved (must be zero) |
| 0x000A | 0x06 | Interrupt vectors |
| 0x0010 | variable | Program contents |

System documentation

The interrupt vectors are the NMI, reset, and IRQ vectors, in that order. They are loaded starting from the address $FFFA. The NMI and IRQ vectors are currently unused and should be set to 0. The reset vector at $FFFC is the main program entry.

The program contents are loaded starting from the address $0400.

Page 2 (addresses from $0200 to $02FF) is reserved for I/O operations. Currently, the following I/O operations are defined:

| Address | Name | Read / Write | Description |
| ------- | -------- | ------------ | ------------------------------------------------------------- |
| $0200 | EXIT | W | Any write here exists the program with the written exit code. |
| $0201 | STDIN | R | Read a byte from the standard input (blocking). |
| $0202 | STDOUT | W | Write a byte to the standard output. |
| $0203 | STDERR | W | Write a byte to the standard error. |
| $0240 | RAND | R | Read a random byte. |
| $0241 | FSTIN | R | Status of stdin: EOF if bit 7 set. |

On program start, all CPU registers will be in an unknown state. In particular, you should remember to clear the decimal flag. page 3 (addresses from $0300 to $03FF) will contain the command line arguments as a null-terminated list of null-terminated strings.

Everything other than the I/O area is RAM, including the command line argument area, the program contents, and the interrupt vectors. Free areas will contain all zeroes.

Executing an undocumented opcode ends the program with exit code 2.

Sample programs

Executables for the sample programs are in the samples directory. The sources are available in src/samples. You need the ca65 assembler from the cc65 package to build them.

| Name | Description |
| ------- | --------------------------------- |
| hello | Prints "Hello, world!" to stdout. |
| cat | Reads stdin and writes to stdout. |
| echo | Prints the arguments to stdout. |
| guess | Guess the number game. |

Ideas for future versions

- Provide a lib6502 implementation (file I/O and more)
- Provide a cc65 library implementation