In assembly-language notation, consider the instruction: 500 SUBA # 50 What is the command (opcode/mnemonic) field in this line, assuming 500 is a sequence/label and '# 50' denotes an immediate operand?

Introduction / Context:
Assembly-language source lines are typically written as a sequence of fields such as an optional label or sequence number, an operation code (opcode or mnemonic), addressing mode markers, and one or more operands. Understanding which part is the command field helps learners read and debug low-level code quickly and accurately.

Given Data / Assumptions:

• The line is written as: 500 SUBA # 50.
• We treat 500 as a sequence/label field rather than an opcode.
• We interpret '# 50' as an immediate addressing operand (literal value 50).
• The command field refers to the opcode/mnemonic that tells the CPU which operation to perform.

Concept / Approach:
Most assemblers parse a line using the format: [label] mnemonic operands [; comment]. The mnemonic is the command field. Addressing modifiers such as '#' indicate the addressing mode (e.g., immediate), while numeric prefixes or sequence numbers at the beginning can serve as labels or line numbers, not as opcodes. Therefore, in the example, 'SUBA' is the command field, indicating a subtraction into register A (exact behavior depends on the architecture).

Step-by-Step Solution:

Verification / Alternative check:
In classic assembly manuals, the opcode appears between an optional label and the operand list. Immediate constants are commonly prefixed with '#' (or similar notation), reinforcing that they are data, not commands.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing operands with opcodes; assuming numbers at the start of a line are instructions; overlooking addressing-mode symbols that modify operands, not mnemonics.

Final Answer:
SUBA