The machine architecture defines the processor state and instructions visible to an assembly program. ivss (Project) implements a hypothetical 32-bit machine with byte-addressed memory, sixteen general-purpose registers, and three control registers. Its emulator fetches one four-byte instruction at a time:
unsigned char bytes[4]; Mem_ReadBytes(bytes, GPR[PC], ASM_INSTR_WIDTH); GPR[PC] += ASM_INSTR_WIDTH;
ASM_INSTR_WIDTH is four. Advancing the program counter before execution makes it point at the following instruction; a branch or call can replace that value. The emulator begins at 0x40000000, with the stack pointer initialized to 0x80000000.
The registers hold operands and addresses. Addressing modes determine how instructions obtain values from them or memory. The instruction format encodes operations, register numbers, and displacements.
Control registers manage interrupts, while the stack holds saved values and return addresses. These are properties of the simulated machine; the host running the emulator need not execute this instruction set natively.
Source: emulator.c.