Microcode

1
Microcode

• Source Digital Computer Electronics (Malvino and
  Brown)

2
Micro-code

• Micro-code is the instructions at the lowest
  level, closest to the hardware.
• Any higher-level instructions (including
  assembly/machine language) must be converted to a
  lower level.
• A single machine-language instruction (like Load
  Accumulator A) typically consists of several
  micro-code instructions.

3
Where is microcode stored?

• It used to literally be wired in (hence the term
  hard wired).
• Typically it stored in ROM.
• If the code is stored in EEPROM, it can be
  changed this is known as microprogramming.
• But this is something one does on a rare
  occasion.
• Sometimes referred to as firmware, an
  intermediate between software and hardware.

4
Machine language

• A level above micro-code.
• The instructions are numbers, which really are
  the addresses of the micro-code instruction in
  ROM.
• Mnemonic version of machine language is called
  assembly language.

5
Assembly is a mnemonic
6
Getting down to hardwares level

• High level programs are translated into assembly
  language or machine language by a compiler.
  Assembly language programs are translated into
  machine language by an assembler.
• Each processor has its own unique machine
  language. Thus code must be rewritten or at least
  recompiled to run on different processor
  (different hardware).

7
A simple design

• Next we will show a computer design that uses the
  basic bus architecture
• A bus is a basically just wires through which
  data travels from one part of a computer to
  another.
• Usually its implied the path is shared by a
  number of parts.
• (One also talks about buses in networks.)

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Bus
Keyboard encoder
Input port 1
Accumulator
Flags
ALU
Input port 2
Prog. counter
TMP
Mem.Add.Reg.
B
Memory
C
MDR
Output port 3
Display
Instr. Reg.
Output port 4
Control
9
Input ports

• Keyboard encoder converts key pressed into
  corresponding string of bits (ASCII)
• Input port 1 where keyboard data is entered,
  usually contains some memory (a buffer) where
  data is held until the processor is ready for it
• Input port 2 where non-keyboard data is entered

10
Program counter

• The program counter points to the current line of
  the program (which is stored in memory)
• This design shows arrows connecting the PC to
  and from the bus, why?
• If the next instruction to be executed is not the
  next line of code in memory, such as
• If
• Loops
• Subroutines, functions, etc.

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MAR, MDR and Memory

• MAR (Memory Address Register) holds the address
  of the memory location being read from or written
  to
• Not necessarily same as program counter
• Memory (RAM) the place where data and
  instructions are stored
• MDR (Memory Data Register) holds the data that is
  being read from or written to memory
• Bi-directional connection to bus for reading and
  writing

12
Instruction Register

• Instruction register holds the instruction that
  is currently being executed.
• A given line of assembly or machine language code
  involves several micro-code instructions, the
  instruction register holds onto the instruction
  until all of the micro-instruction steps are
  completed

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Controller/Sequencer

• Executes the program at the lowest level.
• Sends signals to the control pins of all the
  devices involved.
• These lowest level instructions are in ROM.
• Each assembly-level instruction has a numerical
  counterpart (machine language) the numerical
  counterpart is the address of the microcode for
  that instruction stored in ROM.
• Not shown, controller connects to everything.

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Accumulator and ALU

• Accumulator register used in conjunction with
  the ALU.
• Data upon which arithmetic or logic operations
  will eventually be performed is stored here also
  the results of these are stored here.
• ALU (Arithmetic Logic Unit) where operations that
  change the data (as opposed to just moving it
  around) are done.

15
Flags

• Flags are output from the ALU that are distinct
  from data (data output goes to Acc. A)
• For example,
• A carry from an addition
• An indication of overflow
• These are needed for program control or to
  indicate possible errors
• The result of a logical comparison (lt, gt, )
• These are needed for control (ifs, loops, etc)

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TMP, B and C

• TMP is the other register used in conjunction
  with the ALU the distinction is that answers are
  generally sent to Accumulator A.
• B and C are additional registers used for holding
  data temporarily.
• They allow additional flexibility and reduce the
  amount that must be written to memory.

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Output ports

• Output port a connection to the outside world
• Usually includes a buffer
• This design has to one for displayed output and a
  second for other output (e.g. storage)

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Micro-code

• Let us now examine the steps involved in the
  assembly (machine language) instruction Load
  Accumulator A

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What do you mean by Load

• There are different types of Loads
• Load
• Instruction and address
• Associated data is the address of data that is to
  be put in Acc. A
• Load immediate
• Instruction and data
• Associated data is actual data to be sent
  directly to Acc. A
• Load indirect
• Instruction and address of address
• Associated data is an address. Found at that
  address is another address. Then at that second
  address is the data to be placed in Acc. A.

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Fetch Cycle

• Address State the value of the program counter
  (which recall is the address of line of the
  program to be performed) is put into memory
  address register.
• Increment State the program counter is
  incremented, getting it ready for the next time.
• Memory State the current line of the program is
  put into instruction register (so Control knows
  what to do).

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Execution cycle (Load Acc. A)

• The remaining steps depend on the specific
  instruction and are collectively known as the
  execution cycle.
• Recall the instruction consisted of a load
  command and an address. A copy of the address
  is now taken over to the memory address
  register.
• The value at that address is loaded into
  Accumulator A.
• For the load command, there is no activity
  during the sixth step. It is known as a "no
  operation" step (a "no op" or "nop").

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Bus
Keyboard encoder
Input port 1
Accumulator
Flags
ALU
Input port 2
Prog. counter
TMP
Mem.Add.Reg.
B
Memory
C
MDR
Output port 3
Display
Instr. Reg.
Output port 4
Control
23
Data Movement

• Many of the micro-code steps involve moving data
  and addresses to various locations (registers,
  memory locations, etc.).
• The information is often, but not always, sent
  over the bus.
• So information must be put on and taken from the
  bus.

24
Load and Enable

• With memory, one talks about reading and writing.
• With registers and the bus, one talks about
  enabling and loading.
• Enabling placing a value from a register on the
  bus.
• Loading placing a value from the bus into a
  register.

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Register Control pins

• A register that takes values off of the bus (e.g.
  the Memory Address Register, MAR) will need a
  load control pin
• It does not always take the value on the bus,
  instead it takes the current value on the bus
  when the load pin is activated
• Active highmeans the action is performed when
  the pin is high
• Active low means the action is performed when
  the pin is low

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The clock pin

• The clock is another control pin (sometimes
  called a timing pin) which determines when a
  register takes the value on the bus
• The load input determines if the register takes
  the value
• The clock input determines when the register
  takes the value

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The clock

• A binary clock 10101010101010101010
• Each cycle (01) should take the same amount of
  time (the time for a cycle the period)
• The number of cycles in a second is called the
  frequency
• on the edge many registers load on the clocks
  edge
• Positive edge as 0 goes to 1
• Negative edge as 1 goes to 0

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Enable

• The reverse of a load is an enable, this is when
  a device places a value on the bus
• A register that places values on the bus (e.g.
  the buffer associated with an input port) must
  have an enable control pin
• Again enabling may be active high or active low

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Only one bus driver

• Only one item can place its value on the bus
  (drive the bus) at a time. WHY?
• Suppose two items drive the bus and that they
  have different values
• Then there would be a direct connection (the bus
  is essentially just wire) between a high voltage
  and a low voltage
• Since wire offers little resistance, there would
  be a very large current a.k.a. a short
• Large currents destroy digital circuits

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Three-State logic

• If a device is not driving the bus, it must be
  effectively disconnected from the bus
• Otherwise the short problem
• Thus we need three-state logic
• High
• Low
• Disconnected

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Tri-State buffer

• Disconnecting all devices except the bus
  driver from the bus is done with tri-state
  buffers
• These are not shown in our diagram and are
  distinct from chips
• Thus we wont find the kind of enable control
  pins discussed here on chips

32
Other control pins

• Items involved in data manipulation (as opposed
  to simply data movement) will require additional
  control pins
• For example, the program counter needs to be
  incremented
• Thus additional control pins are required
• These pins are sometimes also referred to as
  enable pins, as they enable a particular action

33
ALU control

• The primary data manipulator is the ALU
• The 74181 is a simple ALU
• It has an M select to choose between logic and
  arithmetic functions (M)
• It has a set of S selects (S0, S1, S2, S3) to
  choose among the various functions of that type

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74181 ALU Chip
Data in
Data out
Data in
Control pins
35
Micro-code is

• Micro-code is 1s and 0s stored in ROM
• The ROM output is connected to control pins
• For example, one micro-code instruction is to
  take the value from the program counter to the
  memory address register
• So send active signals to enable the PC and
  load the MAR