Two-Pass SIC/XE Assembler Simulator

A two-pass assembler processes source code twice. Pass 1 scans the entire program to build the symbol table (mapping labels to addresses) and track the location counter. Pass 2 uses the symbol table to generate machine code (object code), resolving all forward references. This approach elegantly handles symbols that are used before they are defined—a common scenario in assembly programming.

SIC/XE (Simplified Instructional Computer with Extra Equipment) is a hypothetical computer designed for educational purposes. It features up to 1MB of memory (20-bit addressing), 9 registers (A, X, L, B, S, T, F, PC, SW), and supports four instruction formats. It's widely used in computer science courses to teach assembly language, assembler design, and system programming concepts.

In PC-relative addressing, the displacement is calculated as: displacement = target_address - (current_PC), where current_PC = instruction_address + 3 (for Format 3). The displacement is a 12-bit signed value (range: -2048 to +2047). The assembler automatically selects PC-relative mode when the target is within range, setting the p flag to 1 in the instruction encoding.

Format 1 (1 byte): Opcode only—used for instructions like FIX, FLOAT, HIO.
Format 2 (2 bytes): Opcode + two register fields—used for register-to-register operations like ADDR, CLEAR, COMPR.
Format 3 (3 bytes): Opcode + flags (n,i,x,b,p,e) + 12-bit displacement—the most common format.
Format 4 (4 bytes): Same as Format 3 but with a 20-bit absolute address field (e=1), prefixed with + in assembly.

Assembly code often contains forward references—symbols used before they are defined (e.g., a JMP to a label defined later). In a single pass, the assembler wouldn't know the address of the forward-referenced symbol. Pass 1 collects all symbol definitions and their addresses. Pass 2 then has complete information to generate correct machine code for every instruction. Two-pass design is a classic and elegant solution to this fundamental problem.

This simulator supports key SIC/XE assembler directives: START (sets program origin), END (marks program end and entry point), BYTE (declare byte constants like C'ABC' or X'F1'), WORD (declare 3-byte word constants), RESB (reserve bytes), RESW (reserve words), EQU (equate symbol to a value), ORG (change location counter), BASE/NOBASE (control base-relative addressing).