Itech 1000

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CHAPTER 6:
The Little Man Computer

The Architecture of Computer Hardware, Systems Software & Networking:
An Information Technology Approach

5th Edition, Irv Englander

John Wiley and Sons 2013

PowerPoint slides authored by Angela Clark, University of South Alabama

PowerPoint slides for the 4th edition were authored by Wilson Wong, Bentley University

6-*

The Little Man Computer

6-*

Mailboxes: Address vs. Content

Addresses are consecutive starting at 00 and ending at 99
Content may be
Data, a three digit number, or
Instructions

Address

Content

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Content: Instructions

Op code
In LMC, represented by a single digit
Operation code
Arbitrary mnemonic
Operand
In LMC, represented by two digits following the op code
Object to be manipulated

Data or

Address of data

Address	Content
Op code	Operand

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Magic!

Load program into memory
Put data into In Basket

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Assembly Language

Specific to a CPU
1 to 1 correspondence between assembly language instruction and binary (machine) language instruction
Mnemonics (short character sequence) represent instructions
Used when programmer needs precise control over hardware, e.g., device drivers

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Instruction Set

Arithmetic	1xx	ADD
2xx	SUB
Data Movement	3xx	STORE
5xx	LOAD
Input/Output	901	INPUT
902	Output
Machine Control (coffee break)	000	HALT COB

6-*

Input/Output

Move data between calculator and in/out baskets

Content
Op Code	Operand (address)
IN (input)	9	01
OUT (output)	9	02

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LMC Input/Output

OUT

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Internal Data Movement

Between mailbox and calculator

Content
Op Code	Operand (address)
STO (store)	3	xx
LDA (load)	5	xx

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LMC Internal Data

LDA

STO

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Arithmetic Instructions

Read mailbox
Perform operation in the calculator

Content
Op Code	Operand (address)
ADD	1	xx
SUB	2	xx

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LMC Arithmetic Instructions

ADD

SUB

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Data storage location

Physically identical to instruction mailbox
Not located in instruction sequence
Identified by DAT mnemonic

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Simple Program: Add 2 Numbers

Assume data is stored
in mailboxes with
addresses >90
Write instructions

Input a #

Store the #

Input a #

Add

Output the

number

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Program to Add 2 Numbers

Mailbox	Code	Instruction Description
00	901	;input 1st Number
01	399	;store data
02	901	;input 2nd Number
03	199	;add 1st # to 2nd #
04	902	;output result
05	000	;stop
99	000	;data

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Program to Add 2 Numbers:
Using Mnemonics

Mailbox	Mnemonic	Instruction Description
00	IN	;input 1st Number
01	STO 99	;store data
02	IN	;input 2nd Number
03	ADD 99	;add 1st # to 2nd #
04	OUT	;output result
05	COB	;stop
99	DAT 00	;data

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Program Control

Branching (executing an instruction out of sequence)
Changes the address in the counter
Halt

Content
Op Code	Operand (address)
BR (Jump)	6	xx
BRZ (Branch on 0)	7	xx
BRP (Branch on +)	8	xx
COB (stop)	0	(ignore)

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LMC Instruction Set

Arithmetic	1xx	ADD
2xx	SUB
Data Movement	3xx	STORE
5xx	LOAD
BR	6xx	JUMP
BRZ	7xx	BRANCH ON 0
BRP	8xx	BRANCH ON +
Input/Output	901	INPUT
902	OUTPUT
Machine Control (coffee break)	000	HALT COB

6-*

Find Positive Difference of 2 Numbers

00	IN	901
01	STO 10	310
02	IN	901
03	STO 11	311
04	SUB 10	210
05	BRP 08	808	;test
06	LDA 10	510	;if negative, reverse order
07	SUB 11	211
08	OUT	902	;print result and
09	COB	000	;stop
10	DAT 00	000	;used for data
11	DAT 00	000	;used for data

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Instruction Cycle

Fetch: Little Man finds out what instruction he is to execute
Execute: Little Man performs the work

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Fetch Portion of
Fetch and Execute Cycle

1. Little Man reads the address from the location counter

2. He walks over to the mailbox that corresponds to the location counter

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Fetch, cont.

3. And reads the number on the slip of paper (he puts the slip back in case he needs to read it again later)

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Execute Portion

1. The Little Man goes to the mailbox address specified in the instruction he just fetched

2. He reads the number in that mailbox (he remembers to replace it in case he needs it later)

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Execute, cont.

3. He walks over to the calculator and punches the number in

4. He walks over to the location counter and clicks it, which gets him ready to fetch the next instruction

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von Neumann Architecture (1945)

Stored program concept

Memory is addressed linearly

Memory is addressed by location number without regard to content

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