Computer Systems

2009-2010

Week 7: Looping and Input/Output with 3-bit

Alma Whitfield

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Quick Quiz1. How many different instructions

does 3-bit use?

a) 3b) 8c) 10d) 16

3 bits are available for the op-codes with

which 8 different patterns of 0s and 1s

are possible

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Quick Quiz2. How many bits can each memory

location hold?

a) 3b) 8c) 10d) 16

Each location can hold one instruction. Each instruction is 8

bits long

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Quick Quiz3. Each instruction is split into two

sections, the first section is known as what?

a) The operandb) The opinstructionc) The op-coded) The opexecute

Abbreviation for operation

code

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Quick Quiz4. Each instruction is split into two

sections, the second section is known as what?

a) The operandb) The opinstructionc) The op-coded) The opexecute

The part that identifies what the op-code is to act

upon

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Quick Quiz5. Which CPU register contains the

address of the next instruction to be executed?

a) Accumulatorb) Instruction Registerc) Program Counterd) Index Register

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Quick Quiz5. Which CPU register contains values

brought from memory and perhaps added to or subtracted from by ADD and SUB instructions?

a) Accumulatorb) Instruction Registerc) Program Counterd) Index Register

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Quick Quiz5. Which CPU register holds a copy of

the instruction currently being executed?

a) Accumulatorb) Instruction Registerc) Program Counterd) Index Register

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Repetition in 3-bit programs The instructions considered so far

are: LDD LDI STD ADD SUB

None of these enable instructions to be executed repeatedly inside loops

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Repetition and selection in 3-bit programs These next instructions provide

these facilities JMP - repetition JEZ - repetition and selection

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Repetition in 3-bit programs An infinite

loop Demo

ADD 16

START

Flow chart

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JMP JMP is the Jump command. It changes the sequence of

execution of the program by changing the contents of the program counter to the address specified E.g. JMP 10, will cause the next

instruction to be fetched from location with address 10 instead of the next one after the current address

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Repetition and selection in 3-bit programs An finite loop

Demo jez.tbt

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Flow chart

Repetition and selection in 3-bit programs

LDD 16

START

SUB 17

STD 16

AC = 0?

STP

True

False

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JEZ JEZ is similar to JMP but it will only jump if

the AC is equal to zero

Jump if Equal to Zero

Thus selection is determined by whether the accumulator contains zero

All computers make decisions by comparing numbers. 3-bit does it by comparing the number in the accumulator with zero

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JEZ Given the

following program ...

... what will happen?

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We have now covered all eight instructions: the instruction set

Mnemonic Brief meaning

000 STP halt

001 LDD load accumulator direct

010 LDI load accumulator immediate

011 STD store accumulator direct

100 ADD add to accumulator

101 SUB subtract from accumulator

110 JMP jump to next instruction

111 JEZ jump to next instruction if contents of accumulator is zero

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CPU The CPUs processing power is

measured with a clock speed.

The higher the speed the quicker the CPU will process instructions

MHz GHz

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3-bit and the clock speed 3-bit has a clock speed too. By default it is set to 0.3 Hz

1 Hz is a frequency of one instruction per second so 0.3 Hz means each instruction takes about 3

seconds Very slow, this is deliberate so that you can see

the instructions being executed real computers can execute billions of instructions per

second The clock speed can be set to:

0.2 Hz 0.3 Hz 10 Hz 50 Hz

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Inputting and outputting data Outputting Data Inputting Data Using the hard disc

Saving / Opening

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Memory mapped input and output Input concerns getting data into the

computer Output concerns getting data/information

out of the computer 3-bit uses memory mapping to provide

input and output Input is achieved by reading from a

specific memory locations Output is achieved by writing to specific

memory locations

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Memory mapped locationsMemory location

addresses

Purpose

18 - 25 Display memory

26 Sets display mode

27-28 Network interface card

29 Hard disc buffer

30 Printer buffer

31 Keyboard

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Today Location 31 Keyboard Location 30 Printer Location 29 Disk Buffer

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Next Week Location 18 – 25 Display memory Location 26 Screen Mode

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Week after Next Location 27 and 28 Network

Interface Card

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Keyboard LDD 31 will copy what is in

address 31 to the accumulator.

31 is mapped to an input device, in this case the keyboard

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LDD 31 Effectively, LDD 31 will:

ask the user for a value store the value in address 31 copy the value from memory address

31 to the accumulator

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STD 31?Memory Address 31 is mapped to an

input device, what happens if you try STD 31?

a) 3BIT will perform an illegal operation and close down

b) Nothing will happenc) The value in the accumulator will be

copied to address location 31 as normald) 31 will be copied into the accumulator

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LDD 31If there is a value already present in

location 31, will it still ask the user for input or will it use the value already there?

a) Askb) Use

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Outputting: STD 30

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Outputting: STD 30 Use STD 30 to send the contents of

the accumulator to the printer

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Outputting: STD 30 Effectively, STD 30 will:

copy what is currently in the accumulator to memory address 30

copy what is in memory address 30 to the printer

This button will clear any output occurred

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LDD 30?Memory Address 30 is mapped to an output

device, what happens if you try LDD 30?

a) 3BIT will perform an illegal operation and close down

b) Nothing will happenc) The value in the accumulator will be copied to

address 30d) The value in memory address 30 will be copied

into the accumulator

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The Hard DiscYour 3-bit programs can

be saved and loaded as required

You can also edit and open data files

Contents of the data file are shown in this window

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LDD 29 Data file input memory is mapped

to: Location 29

Data files work using positional pointers

For the purpose of 3-bit, the pointer starts at the beginning of the file and moves down one line each time data is fetched

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Data LDD 29

This will fetch data from the data file Put the data into memory address 29 Copy the value in location 29 into the

accumulator

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STD 29?Memory Address 29 is mapped to an input

device, what happens if you try STD 29?

a) 3BIT will perform an illegal operation and close down

b) Nothing will happenc) The value in the accumulator will be

copied to address 29 as normald) 29 will be copied into the accumulator

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What ends up in the AC?0 LDD 291 STD 302 STP

1533650

a) 29b) 0c) 15d) 30

Data File

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What about now?0 LDD 291 STD 302 JEZ 43 JMP 04 STP

a) 29b) 0c) 15d) 30

1533650

Data File

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What have we covered?

STD 30 sends to printer LDD 31 gets data from keyboard We can save programs to hard disc We can load programs from hard

disc We can read a data file with LDD 29

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3-bit is not so complicated?