Little Man Computer

Introduction

The LMC is a simplified assembly language designed for teaching about the Fetch-Execute cycle, computer architecture and assembly. It was created by Dr. Stuart Madnick in 1965. See 101 Computing for details and challenges. See Peter Higginson's page for even more details.

There are two simulators that we'll use:

Peter Higginson has also made a version that is close to the AQA exam board specifications.

Exercises:

101 Computing LMC Challenges Template Doc

This is to practice using pseudocode to design algorithms; translation (compilation) into Assembly; Executing & debugging code at the level of the Fetch-(Decode)-Execute cycle.

The LMC Mnemonics and Op Codes

Notes:

they are decimal op codes
the xx refer to a memory address
BRP is branch positive or zero

Translations from Pseudocode to LMC

Remember that one High-Level-Language instruction can translate to many Low-Level-Language instructions.

Declare & initialize a variable

num ← 4

Is equivalent to allocating memory with a label

num DAT 4

Input a variable

INPUT num

Is equivalent to inputting and then storing (assume num is a valid data label)

INP

STA num

Output a variable

OUTPUT num

Is equivalent to loading and then printing (assume num is a valid data label)

LDA num

OUT

Add one to a variable

num ← num + 1

Is equivalent to Load, Modify, Store (assume num and one are valid data labels)

LDA num

ADD one

STA num

In assembly, all selection and iteration flow control is implemented using branch commands that can depend on the ALU flags. In the LMC we have:

BRA = Branch Always
BRZ = Branch if ACC stores zero
BRP = Branch if ACC stores a positive number

IF Statement / Binary Selection

IF num1 > num2 THEN

  PRINT num1

ELSE

  PRINT num2

END IF

Translates to (assuming num1 and num2 already valid data labels)

         LDA num1

         SUB num2

         BRP bigger

smaller  LDA num2

OUT

         BRA endif

bigger   LDA num1

OUT

endif    HLT

While-Do and Repeat-Until loops: See count down examples below.

Example Programs

A + B

INP

STA 99 // A

INP

STA 98 // B

ADD 99

OUT

HLT

B - A

INP

STA 99 // A

INP

STA 98 // B

SUB 99

OUT

HLT

A - B

INP

STA 99 // A

INP

STA 98 // B

LDA 99

SUB 98

OUT

HLT

Note that in A-B we had to store both A and B then load A back into the Accumulator before subtracting B

Count Down v1

Simple version with a WHILE loop

INPUT count

WHILE count <> 0 DO

  OUTPUT count

  count ← count - 1

END WHILE

Here is an equivalent LMC assembly program with a manually referenced memory address for the count

INP

        STA 99   // count

loop    BRZ end

        LDA 99

OUT

        SUB one

        STA 99

        BRA loop

end     HLT

one     DAT 1

Note that as only count is ever in the Accumulator, we can simplify to get

INP

loop    BRZ end

OUT

        SUB one

        BRA loop

end     HLT

one     DAT 1

But in most cases you'd do more inside the loop and such a simplification is not possible

Count Down v2

Simple version with a REPEAT-UNTIL loop

INPUT count

REPEAT

  OUTPUT count

  count ← count - 1

UNTIL count <= 0

Here is an equivalent LMC assembly program with a labelled memory address for the count

INP

        STA count

loop    OUT

        LDA count

        SUB one

        STA count

        BRP loop

HLT

one     DAT 1

count   DAT

Note that as only count is ever in the Accumulator, we can simplify to get

INP

loop    OUT

        SUB one

        BRP loop

HLT

one     DAT 1

But in most cases you'd do more inside the loop and such a simplification is not possible

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