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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