The Little Man Computer

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The Little Man Computer

• LMC was created by Dr. Stuart Madnick at MIT 1965

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Layout of the LMC

• LMC consists of a walled mailroom
• There are 100 mailboxes addressed from 00 to 99
• A mailbox can hold a signal slip of paper which
  is written three-digit decimal number
• Calculator can add, subtract and hold 3-digit
  temporarily number
• Hand counter is for little man to see which
  mailbox he has to get reset button is for people
  outside mailroom to press
• Basket in and Basket out are for little
  communicate with outside world

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Figure 6.1 The Little Man Computer
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Operation of the LMC

• 3 digits mean following
• 1 digits for instruction (op code)
• The rest for mailbox address (address)

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Operation of the LMC cont.

• Load instruction op code 5
• Store instruction op code 3
• Add instruction op code 1
• Subtract instruction op code 2
• Input instruction op code 9, address 01
• 901
• Output instruction op code 9, address 02
• 902
• Coffee break instruction (Halt) op code 0

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Load
Walk to mailbox address
Little Man
Read 3-digit number in that mailbox
Walk to calculator and put these numbers to the
calculator
The numbers in the mailbox are left unchanged
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Store
Walk to calculator and read 3-digit number on
calculator
Little Man
Write 3-digit number in a blank paper
Walk to mailbox address and put this paper to the
mailbox
The numbers in the calculator are left unchanged
8
Add
Walk to mailbox address
Little Man
Read 3-digit number in that mailbox
Walk to calculator and add these numbers to the
calculator
The numbers in the mailbox are left unchanged
9
Subtract
Walk to mailbox address
Little Man
Read 3-digit number in that mailbox
Walk to the calculator and subtract value on
calculator from mailbox value
The numbers in the mailbox are left unchanged
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Input
Walk basket in
Little Man
Read 3-digit number in that basket
Walk to the calculator and put these numbers on
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Output
Walk to the calculator
Little Man
Read 3-digit number from the calculator write
down in a paper
Walk to the basket out and put this paper in
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Coffee break (halt)
Little Man

• Summary of instructions
• Move data
• Load, store
• Simple arithmetic
• Add, subtract
• Input and output
• Input, output
• Control machine
• Coffee break

Stop doing anything Just relax and take a glass
of coffee
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A simple program

• We need to find the place for store the program
• We assume that the program is stored at mail box
  address 00
• When LM completes the instruction, he walks to
  the counter to increase its value 1 more
• We can reset LM to execute program by pressing
  the reset button

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Add two numbers

• User places two numbers in the in basket
• Result will be returned in the out basket
• Input 901
• Store 99 399
• Input 901
• Add 99 199
• Output 902
• Coffee break 000

In class Do add 3 number
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An extended instruction set

• Branch unconditionally instruction (Jump) op
  code 6
• Branch on zero instruction op code 7
• Branch on positive instruction op code 8

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Branch unconditionally
Walk to counter
Little Man
Change counter to specific number
Execute the instruction where counter shows
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Branch on zero
Walk to calculator
Little Man
The number on calculator is zero?
Yes, change value in counter to mailbox address
No, dont care
continue executing next instruction
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Branch on positive
Walk to calculator
Little Man
The number on calculator is gt 0?
Yes, change value in counter to mailbox address
No, dont care
continue executing next instruction
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Example of branch program

• While (i0)
• do something
• 45 LDA 90 590 90 contains i
• 46 BRZ 48 748 branch on zero
• 47 BR 60 660
• do something
• 59 BR 45 645
• 60 COB stop

Look at table 6.3 for mnemonic instruction page
155
90 000
90 100
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Another example of branch

• Find the positive difference between two numbers
  ? a b where a, b gt 0
• 00 IN 901
• 01 STO 10 310 store a
• 02 IN 901
• 03 STO 11 311 store b
• 04 SUB 10 210 b-a
• 05 BRP 08 808 gt0?
• 06 LDA 10 510
• 07 SUB 11 211
• 08 OUT 902 print output
• 09 COB 000
• 10 DAT 000 a
• 11 DAT 000 b

In high level language if (b-agt0) print
(b-a) else print (a-b)
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The Instruction Cycle

• Fetch
• Execute
• Von Neumann architecture
• Stored program concept
• Sequential numeric address executing

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Fetch
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Execute
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More example

• Write a Little Man program that accepts three
  values as input and produces the largest of the
  three as output
• Write a Little Man program that prints out the
  odd numbers from 1 to 99. No input is required

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Addressing mode

• Immediate
• Direct
• Indirect
• Register
• Register Indirect
• Indexed Addressing

Why do we need to have other methods of
addressing techniques in stead of only absolute
addressing?
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Immediate addressing

• Data is in the instruction
• No memory reference to fetch data
• Fast
• Limited range

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Direct Addressing

• Address field contains address of operand
• e.g. ADD A
• Add contents of cell A to accumulator
• Look in memory at address A for operand
• Single memory reference to access data
• No additional calculations to work out effective
  address
• Limited address space

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Direct Addressing Diagram
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Indirect Addressing Diagram

• Memory cell pointed to by address field contains
  the address of (pointer to) the operand

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Indirect Addressing

• Large address space
• 2n where n word length
• Multiple memory accesses to find operand
• Often used in branch instructions
• Often used in access table
• Could have multiple levels of indirection
• Like pointer in C or Pascal
• Hence slower

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Little Man indirect addressing(1)
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Little Man indirect addressing(2)
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Register Addressing (1)

• Very fast performance
• Example, move r1, r2

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Register Addressing (2)

• Operand is held in register named in address
  filed
• Limited number of registers
• Very small address field needed
• Shorter instructions
• Faster instruction fetch
• No memory access
• Very fast execution
• Very limited address space
• Multiple registers helps performance
• Requires good assembly programming or compiler
  writing

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Register Addressing Diagram
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Register Indirect Addressing

• Operand is in memory cell pointed to by contents
  of register R
• Large address space (2n)
• n is number of bits in the register
• One fewer memory access than indirect addressing
• Can use Auto incrementing or auto decrementing
  for table access

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Register Indirect Addressing Diagram
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Indexed Addressing (1)
Modifying an address with an index register
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Indexed Addressing (2)
Indexing a base offset address