Crossassemblers

Crossassemblers are software tools that assemble assembly language source code into machine code for a processor architecture different from the one on which the tool runs. They operate as part of cross-development toolchains, allowing developers to write and test code on a host computer while producing binaries for a target device or emulator. This is common in embedded systems, consumer electronics, and retro computing, where the target hardware may be unavailable or impractical to use directly.

A crossassembler must understand the target architecture's instruction set, registers, addressing modes, and endianness. It typically provides architecture-specific directives and macros, enabling portable, single-source code across multiple targets. The output is usually an object file or an intermediate form that is fed to a cross linker and, finally, to a target runtime or ROM image. In practice, crossassemblers are often used together with a cross compiler and debugger, allowing end-to-end development on one machine.

Historically, crossassemblers emerged in the early microcomputer era to support development for devices such as the 6502, Z80, and later ARM and MIPS, from standard personal computers. Today they remain essential for hobbyist retro programming and professional embedded development. Notable examples include CA65, the 6502-oriented cross-assembler used with the cc65 suite, and ACME, a portable cross-assembler for several CPUs. Many toolchains also rely on GNU binutils' assembler for cross-targeting or on vendor-provided assemblers.

Related topics include cross-compilers, cross-linkers, embedded systems, and retro computing.