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Microcode

Source: Digital Computer Electronics (Malvino and Brown)

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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.

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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.

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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.

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Assembly is a mnemonic

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Getting down to hardware’s 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).

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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 it’s implied the path is shared by a number of

parts.– (One also talks about buses in networks.)

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Input port 1 Accumulator

ALU FlagsInput port 2

Prog. counter

Mem.Add.Reg.

Memory

MDR

Instr. Reg.

Control

C

B

TMP

Output port 3

Output port 4

Display

Keyboard encoder

Bus

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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 itInput port 2: where non-keyboard data is entered

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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 MemoryMAR (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 storedMDR (Memory Data Register) holds the data that is being read from or written to memory– Bi-directional connection to bus for reading and

writing

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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.

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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 (<, >, =)• 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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Input port 1 Accumulator

ALU FlagsInput port 2

Prog. counter

Mem.Add.Reg.

Memory

MDR

Instr. Reg.

Control

C

B

TMP

Output port 3

Output port 4

Display

Keyboard encoder

Bus

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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.

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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 high”means 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 valueThe clock input determines when the register takes the value

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The clock

A binary clock: 10101010101010101010Each 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 clock’s 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 pinAgain 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 chipsThus we won’t find the kind of “enable” control pins discussed here on chips

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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

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ALU control

The primary data manipulator is the ALUThe 74181 is a simple ALUIt 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

U1

74181N

~A02

~B01

M8CN7

S06

~A123

~B122

~A221

~B220

~A319

~B318

S15

S33S24

~G 17~P 15

CN4 16AEQB 14

~F1 10

~F3 13~F2 11

~F0 9Data in

Data in

Control pins

Data out

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Micro-code is

Micro-code is 1’s and 0’s 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”