2' Inside A Computer

1
2. Inside A Computer

• Basic functional units of a computer.
• Memory, I/O and the clock.
• Central Processing Unit (CPU).
• Connecting the units together.
• Basic architecture of JASPer.
• Registers.
• Register transfer language (RTL).
• Memory organisation.
• Bytes, words and addresses.
• Burrell 1, 5.1,6.1,7.1.
• Clements 1, 5 (Intro), 5.1, 5.2 (Intro), 5.2.1,
  5.2.2, 5.2.3, 8 (Intro).

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Basic Functional Units Of A Computer
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Memory, I/O The Clock

• Memory unit records the program and data.
• I/O Unit
• Input unit(s) read in programs and data.
• Keyboards, EPS machines, mice etc.
• Output unit(s) write out results and (usually)
  error messages.
• Screens, printers, speakers etc.
• Clock synchronises the actions of the other
  units.

4
Central Processing Unit

• CPU.
• Performs the basic steps of our programs.
• Basic steps are called instructions.
• Instructions specify
• Operation to be performed
• Where operands are in memory.
• Where the result is to be stored.
• CPU has two major components
• Arithmetic logical unit (ALU).
• Control unit (CU).

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Central Processing Unit II

• ALU performs the basic data modifications
  required by our programs.
• Add, subtract, and, or etc.
• CU co-ordinates the actions of the other units.
• Uses the PC to keep track of where the machine
  has got to in the program.
• Manages fetching of instructions and data from
  memory and storing of results in memory.
• Initiates input and output.

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Connecting The Units Together

• A link between two units is called a bus.
• Private connections between each pair of units
  are not necessary.
• Several units may share a connecting link.
• Computers need buses for data, control and other
  signals.
• The bus structure (i.e. the actual system of
  connections) is a major feature of the
  architecture.

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Schematic View Of JASPer
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Schematic View Of JASPer II

• CPU Control Unit ALU Data Buses Registers
  cache RAM.
• Registers are used to store information
• Hold bit patterns.
• Information can be written to / read from them.
• Registers are on the CPU chip so access to them
  is much faster than to memory (1000 times
  faster).
• Some registers are general purpose (totally
  programmer controlled) and some are special
  purpose (partially programmer controlled).
• Usually the more registers a CPU has the more
  modern it is.
• Especially GP registers.

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General Purpose Registers

• Early CPUs had only 1 GP register the
  accumulator.
• JASPer has 2 GP registers A and B.
• Can be used for anything.
• M68K has 32.
• 16 data registers.
• Can be used for anything.
• 16 address registers.
• Can only be used to hold memory addresses
  (pointers).
• More about memory later.

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Special Purpose Registers

• Program counter (PC).
• Holds the address of the next instruction to be
  executed.
• Partially programmer controlled via branching
  instructions.
• JASPer only uses LS 12 bits of PC. MS 4 bits
  ignored.
• M68020 uses all 32 bits of PC (lots more memory).
• Processor Status Register (PSR).
• Only uses LS 4 bits Other bits for interrupts and
  future expansion).
• Flags which are set to 0 or 1 as a side effect of
  various ASP instructions (i.e. partially
  programmer controlled).
• N Negative. Z Zero. V oVerflow. C
  Carry.
• Can be tested to control branches.

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Special Purpose Registers II

• ALUx holds left hand operand for ALU
  operation.
• ALUy holds right hand operand for ALU
  operation.
• R holds result of ALU operation.
• MAR holds address for data transfer to / from
  memory.
• MDR holds data transferred to / from memory.
• INC used to increment PC.
• IR holds instruction currently being executed.

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Register Transfer Language (RTL)

• Notation for explaining the semantics of computer
  operations.
• 123 Numeric constant 123.
• reg Register reg.
• M(adr) Memory location adr.
• X Contents of X.
• Copy (C / Java ).
• Assignment (C / Java ).
• Example
• C x y z
• RTL M(x) M(y) M(z)

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Register Transfer Language II

• Sub-fields of registers are selected as follows
• IR(op-code) Operation code part of IR.
• Memory transactions are specified as follows
• M MAR MDR Write.
• MDR MMAR Read.
• RTL is a de facto standard for describing machine
  instructions.
• Syntax is somewhat variable.

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Memory

• Memory required for
• Instructions and operands of currently executing
  program. This is called primary or main or
  immediate access memory.
• Long term storage of programs etc. in files. This
  is called secondary memory, backing store ...
• Primary memory must
• have fast read/write time (access or cycle time).
• have same access time for all items (random
  access).
• Secondary memory must be BIG.
• Dont need fast or random access.

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Primary Secondary Memory

• Primary memories are electronic devices (chips
  usually).
• Cycle times 10-10 Þ 10-12 seconds.
• Price per bit is comparatively large.
• Secondary memories are usually magnetic devices
  (disks, tapes, CD ROM etc.)
• Cycle times 10-5 seconds (very slow).
• Price per bit is comparatively small.

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Primary Memory Organisation Bytes

• Memory chip made up of millions of bistables,
  each representing one bit.
• Convenient to organise bits in patterns of
  various sizes.
• Smallest grouping is the byte which is (normally)
  8 bits.
• A byte can hold 28 256 different bit patterns.
• A byte is the smallest unit that can be
  transferred to or from the memory in one access.
• NB JASPer has a 16 bit word and is not byte
  addressable.
• More later.

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Primary Memory Organisation Words

• Made up of a number of bytes which are
  consecutive in memory.
• Word size varies from chip to chip
• Bytes Bits Number Of Patterns
• 2 16 64K
• 4 32 4096M
• 8 64 1.8 1019
• A word is the largest unit that can be
  transferred to or from the memory in one access.
• For n bit word memory bus must have n wires.
• Word length fixes memory (and I/O) bus width.
• Word length fixes register width.

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Primary Memory Organisation Addresses

• Programmers specify particular memory locations
  using numerical addresses.
• In effect
• ByteSize memory
• Usually a word must start on a even address as
  words are even numbers of bytes.
• Operands can be bytes or words (sometimes
  longwords).
• Operands are specified by the address of their
  first byte.

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JASPer Memory Organisation Addresses

• Programmers specify particular memory locations
  using numerical addresses (0-FFF).
• In effect
• Word4096 memory
• JASPer uses a 16 bit word.

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Summary

• Functional units memory, I/O, clock, CPU ( ALU
  control).
• Remembering, communicating, synchronising,
  working.
• ALU performs basic data modifications. Control
  unit controls behaviour of CPU units.
• Bus connection between 2 (or more) units.
• ASP registers
• GP A and B.
• SP ALUx, ALUy, R, PC, INC, IR, MAR, MDR, PSR.
• RTL is a way of specifying semantics of
  operations.

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Summary II

• Memory used to hold programs and data.
• Primary memory has fast access time and random
  access. Usually chips.
• Secondary memory is slow and non-random access
  but its big. Usually magnetic devices.
• Byte is 8 bits. Smallest addressable item.
• Smallest addressable item on WinASP is 16 bit
  word.
• Word is 2, 4 or 8 bytes. Largest addressable
  item.
• Bytes in primary memory are specified via
  numerical addresses.