Assembler syntax

MB8 assembly is compiled by the Rust asm crate in this workspace.

Writing a program for the VM

User images start at 0x1000, while kernel and kernel-test images start at 0xE000. Declare that base once at the top of each root source:

.origin 0x1000

start:
    LDI R1, 0x03
    HALT

.addr 0x2000

The final .addr pads the image to its 4 KiB boundary.

Instructions and operands

Instructions and registers are conventionally uppercase. Separate operands with commas and write register pairs with a colon:

LDI R2:R3, MESSAGE
LD R1, [R2:R3]
ST [0xF001], R1
CALL [0xE500]

The assembler provides the ISA and pseudo-instructions directly. Pseudo-instructions include INC, DEC, INC16, MUL, MEMCPY, STRCMP, immediate jumps, and absolute calls, loads, stores, and jumps; no rule-file include is needed.

Directives

• .origin <address> sets the image base and may appear once in the root program.
• .addr <address> pads with zero bytes up to an absolute address.
• .data <byte>, ... emits bytes.
• .ascii "text" emits a string; \n, \0, \\, and \" escapes are supported.
• .include "path.asm" inserts another source relative to the containing file.
• .const @NAME, <value> defines a constant; refer to it as @NAME.

Labels beginning with _ are local to the preceding non-local label in the same source:

.const @SYS_WRITE, 0x02

write:
    LDI R1, @SYS_WRITE
_loop:
    JR [_loop]

Numeric literals use hexadecimal notation. Character immediates should be written as their byte value, such as 0x0A for newline.

Building and running

Assemble one source directly:

cargo run --quiet -p asm -- user/sh.asm -o user/sh.bin

Use make, make kernel, make user for the repository images, and make run to launch the VM.