20-Sep-0194.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001 94.201.lecture7-8-compstate 1 Machine Organization Systems (including

20-Sep-01 94.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 200194.201.lecture7-8-compstate

1

Machine Organization

Systems (including computers) often have STATE-DEPENDENT behaviour

response to an event at some point in time depends on previous sequence of events

current state: the net accumulated effects of the history of events

state-based models use state variables to encode state information

(have you seen this elsewhere?)


2

Machine Organization

Example: combination lock (e.g. on lockers)• combination sequence: X, Y, Z

• go right past X twice, then stop at X

• go left past Y once, then stop at Y

• go right and stop at Z

model of the lock:• input events: position of dial

• output events: clasp locked/unlocked


3

Combination Lock

To describe behaviour: need state variables to remember the relevant history of events; E.G.: • direction of turning dial, • number of times gone past “next combination”

number• “next combination” number expected• whether clasp is locked/unlocked


4

State Variables

statevariables

input

• reaction to an input event may change the output and/or state variable values

output


5

Programmer’s Model

Recall Programmer’s Model processor, memory, I/O communicate via bus inputs and outputs at connected devices need state variables to model behaviour: processor: registers memory: cells I/O components: ports


6

Our Computer System Model

Registers: 16-bits wide Cells: 8-bit contents, 16-bit address Address ports: read/write 8-bit values, 16-bit

address Model behaviour of components in terms of state

variables


7

Programming at the Machine Level

at single instruction level:• instructions are executed by processor

• instructions manipulate state variables

• read, modify, write values

once state variables and instructions are understood, then all of the building blocks available to programmers are known!

there is nothing else !! (not rocket science)


8

Programming at the Machine Level

Program sequences of instructions to generate outputs in response to inputs manage information saved in state variables the intellectual & creative part!


9

encoding details discussed later

Stored Program Concept

1. instructions are encoded into binary values

2. encodings of instructions are loaded into memory

3. processor retrieves and executes instructions from memory (one at a time)

instructions encodings have variable length• 1 to 6 bytes

• not every binary value 1 to 6 bytes long corresponds to an instruction

different!


10

encoded instructions are indistinguishable from encoded information ! (same binary values!)

Programs Internally

instructions

1 to 6 bytebinary values


11

Computer System

Processor

Memory

I/O

Bus

Connected Devices

keyboardmousedisplayprinterdisk drivescommn linksetc.

registers

cellsinstructions

& data

ports


12

Behaviour of Memory

Memory (8-bit cells with 16-bit addresses)• contents of cells are persistent

• contents only change as result of write

• contents before first write? (power on?)

• 16-bit addresses: 216 different addresses

• 64K address space

– 1K = 210 = 102410; 216 = 26 210 = 26 K = 64 K

– 1M = 220 = 1024 K; 1G = 230 = 1024 M

• 64Kb 64 kilobits; 64KB 64 kilobytes


13

Behaviour of Memory

Notation for memory-related behaviour: write(value, addr)

mem[addr] := value read( addr )

mem[addr]


14

Some Types of MemoryRAM: Read-And-Modify a.k.a. Random Access Memory can read and write cell contentsROM: Read-Only-Memory contents “burned in” when manufactured can’t write new valuesPROM: Programmable Read-Only-Memory contents can be programmed once using PROM

Programmer (special device)EPROM: Erasable Programmable ROM program using PROM programmer can be “erased” by UV exposure EEPROM: Electrically Erasable PROM reprogram in-circuit! (power-off persistence!)


15

A Memory Problem

How can binary values wider than 8-bits be stored in 8-bit memory cells ?

Use more than one cell!• use consecutive memory cells

• cannot “address” individual bits in a cell

• e.g. 2 cells 16-bit values


16

Endian Memory Storage

New Problem for 16-bit Values: Endian Scheme• 2 bytes of value stored in 2 cells

• 16-bit value: which cell should hold which byte?

big endian: (Motorola) the least significant byte at the high address

consecutive memory cells

msbyte lsbyte16-bit value?


17

Little Endian Memory Storage

little endian (Intel) the most significant byte at the high address

lsbytemsbyte lsbyte

memorycontents address

msbyte

X

X + 1

16-bit value


18

Little Endian Memory Storage

Little Endian Example: Ouch!

Value = 0F23h 23h

0fh 32ADh

32ACh

Documents

20-Sep-0194.201 - Fall 2001: copyright ©T. Pearce, D. Hutchinson, L. Marshall Sept. 2001 94.201.lecture7-8-compstate 1 Machine Organization Systems (including