hello world in 6502 assembly with memory mapped linux syscalls

hello-world-linux-6502.s

// = hello world in 6502 assembly with memory mapped linux syscalls
// :author: wallabra
// :email: [email protected]
//
// literate 6502 assembly with asciidoc comments
//
// * uses Linux syscalls
//
// addresses exposed by 6502 VM::
// +
// --
// `$10-$1F`:: syscall arguments (4 bytes each)
// `$20-$2F`:: interrupt buffer (4 byte 'interrupt message type' + other data)
// --

.data:

// == write 

// `.string` returns the address
HelloWorld: .string "Hello World"

// assume `LenOf` is the assembler's length function.
//
// assume little endian.
HelloWorldLen: .word !LenOf(HelloWorld)

.text:
_begin:
// select `write` syscall (Linux: 1)
LDA #1
STA $10

// select stdout
LDA #1
STA $14

// data address pointer
//
// (assume it is auto-mapped to the 6502 address space
// in the syscall wrapper)
LDA HelloWorld
STA $18
LDX #1
LDA HelloWorld,X
STA $1

// pass the length of string
LDA HelloWorldLen
STA $1C // LSB; assume little endian
LDX #1
LDA HelloWorldLen,X
STA $1D // MSB

// state that this interrupt reason is a syscall (6502 VM
// interrupt handler)
LDA #$10
STA $20

// trigger interrupt
BRK

// end of program
//
// your 6502 VM may not require an explicit call to `exit` at the end,
// but let's say that it does for the sake of completeness

// select `exit` syscall (Linux: 0x80)
LDA #$80
STA $10

// exit code
LDA #0
STA $14

// state that this interrupt reason is a syscall
LDA #$10
STA $20
BRK