Little Man Computer - Commands - Instructions

Instructions

Each LMC instruction is a 3 digit decimal number. The first digit represents the command to be performed and the final two digits represent the address of the mailbox affected by the command.

Instructions
Numeric code Mnemonic code Instruction Description
1xx ADD ADD Add the value stored in mailbox xx to whatever value is currently on the accumulator (calculator).
Note: the contents of the mailbox are not changed, and the actions of the accumulator (calculator) are not defined for add instructions that cause sums larger than 3 digits.
2xx SUB SUBTRACT Subtract the value stored in mailbox xx from whatever value is currently on the accumulator (calculator).
Note: the contents of the mailbox are not changed, and the actions of the accumulator are not defined for subtract instructions that cause negative results - however, a negative flag will be set so that 8xx (BRP) can be used properly.
3xx STA STORE Store the contents of the accumulator in mailbox xx (destructive).
Note: the contents of the accumulator (calculator) are not changed (non-destructive), but contents of mailbox are replaced regardless of what was in there (destructive)
5xx LDA LOAD Load the value from mailbox xx (non-destructive) and enter it in the accumulator (destructive).
6xx BRA BRANCH (unconditional) Set the program counter to the given address (value xx). That is, value xx will be the next instruction executed.
7xx BRZ BRANCH IF ZERO (conditional) If the accumulator (calculator) contains the value 000, set the program counter to the value xx. Otherwise, do nothing.
Note: since the program is stored in memory, data and program instructions all have the same address/location format.
8xx BRP BRANCH IF POSITIVE (conditional) If the accumulator (calculator) is 0 or positive, set the program counter to the value xx. Otherwise, do nothing.
Note: since the program is stored in memory, data and program instructions all have the same address/location format.
901 INP INPUT Go to the INBOX, fetch the value from the user, and put it in the accumulator (calculator)
Note: this will overwrite whatever value was in the accumulator (destructive)
902 OUT OUTPUT Copy the value from the accumulator (calculator) to the OUTBOX.
Note: the contents of the accumulator are not changed (non-destructive).
000 HLT HALT Stop working.
DAT DATA This is an assembler instruction which simply loads the value into the next available mailbox. DAT can also be used in conjunction with labels to declare variables. For example, DAT 984 will store the value 984 into a mailbox.

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