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Two Pass Assembler

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A Two Pass Assembler translates an assembly language program into machine code using two passes over the source code:

  • Pass 1: Analyzes and collects data like labels, symbols, and literal addresses.
  • Pass 2: Uses the collected data to generate actual machine instructions.

🧾 Pass 1 of the Assembler

The purpose of Pass 1 is to assign memory locations to each instruction and data-defining pseudo-instruction, thereby defining values for symbols that appear in the label fields of the source program.

Initially, the Location Counter (LC) is set to the starting address of the program (usually relative address 0). The assembler then reads a source statement.

🔍 Instruction Processing

  • The operation-code field is examined to determine whether it is a pseudo-op.

  • If it is not a pseudo-op, the Machine Op-Code Table (MOT) is searched to match the op-code.

  • The matching MOT entry provides the length of the instruction (typically 2, 4, or 6 bytes).

  • The operand field is scanned for literals. If a new literal is found, it is added to the Literal Table (LT) for later processing.

  • The label field is then examined:

    • If a symbol is present, it is entered into the Symbol Table (ST) along with the current value of the location counter.

  • The location counter is incremented by the instruction length.

  • A copy of the source line is saved for use by Pass 2.

This process is repeated for each source statement.

⚙️ Handling Pseudo-Operations (Pseudo-Ops)

Pass 1 handles only those pseudo-ops that either:

  • Define symbols, or

  • Affect the location counter

✅ USING and DROP

  • These are simply saved for Pass 2.

  • Pass 1 does not perform any processing beyond saving the cards.

✅ EQU

  • Pass 1 uses this to define a symbol in the label field.

  • It evaluates the expression in the operand field to determine the symbol’s value.

✅ DS and DC

  • These pseudo-ops can:

    • Define symbols, and

    • Affect the location counter

  • The operand field is examined to determine the amount of storage required.

  • Due to alignment requirements (e.g., full words must start at addresses that are multiples of 4), the location counter may be adjusted before assigning the address.

🔚 End of Pass 1

When the END pseudo-op is encountered, Pass 1 is terminated.

Before transferring control to Pass 2, the assembler performs housekeeping tasks, such as:

  • Assigning locations to literals collected during Pass 1.

    • This is similar to processing DC pseudo-ops.

  • Reinitializing conditions and data structures required for Pass 2.

🧾 Pass 2 of the Assembler: Code Generation

After Pass 1 has defined all the symbols, Pass 2 completes the assembly process by:

  • Generating machine code for each instruction.

  • Formatting the code for loader compatibility.

  • Printing an assembly listing that includes both the source statement and the corresponding hexadecimal machine code.

🏁 Initialization

  • The Location Counter (LC) is re-initialized, just like in Pass 1.

  • Processing starts by reading each source statement saved during Pass 1.

🔍 Instruction Processing

  1. Identify Pseudo-ops:

    • If the statement is a pseudo-op, it is handled separately (explained below).

    • If it is not a pseudo-op, the Machine Op-Code Table (MOT) is searched to:

      • Identify the instruction format (e.g., RR, RX),

      • Get the binary op-code, and

      • Determine the length of the instruction.

  2. Operand Processing:

    • RR-format (Register-to-Register):

      • Both register fields are evaluated and inserted into their respective 4-bit fields.

    • RX-format (Register and Storage):

      • Register and index fields are evaluated similarly.

      • The effective address (EA) is calculated from the operand field.

      • The base and displacement fields are computed using the Base Table (BT).

  3. Code Generation:

    • The assembled instruction is formatted for the loader.

    • Several machine instructions may be grouped into a single output card.

    • A listing line is printed that includes:

      • The source statement,

      • Its assigned memory location,

      • And the generated machine code in hexadecimal.

    • The Location Counter is then incremented accordingly.

⚙️ Pseudo-op Processing

  • EQU:

    • Requires minimal processing.

    • Symbol definition was already completed in Pass 1.

    • Simply printed in the assembly listing.

  • USING and DROP:

    • Were ignored in Pass 1 but are important in Pass 2.

    • The operand fields are evaluated.

    • The corresponding Base Table (BT) entry is:

      • Marked as available if USING,

      • Marked as unavailable if DROP.

  • DS and DC:

    • As in Pass 1, the location counter is updated based on required storage.

    • DC (Define Constant) also requires actual code generation in Pass 2:

      • May involve conversions (e.g., character → binary, float → machine format),

      • May include evaluations of address constants or symbolic values.

🧾 Handling Literals

  • At the end of the source program (upon encountering the END pseudo-op):

    • Remaining literals in the Literal Table (LT) are processed.

    • Code is generated for each literal, similar to DC.

✅ Final Output

  • All machine instructions are formatted for the loader.

  • The complete assembly listing is printed.

  • The program is now ready for loading and execution.

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