MicroProgramming
Programming at a different
level
Copyright © 2005 – Curt Hill
The Control Unit
• What does it do?
• Decodes the instruction
• Causes the ALU to perform the needed
actions
• This can be done in two ways:
– Hardwire the CU from gates
– Microprogram the CU
Copyright © 2005 – Curt Hill
History 1
• Control Units were traditionally
hardwired
• Wilkes proposed in papers in 1951 and
1953 the concept of a microprogram
• This is the same Wilkes of the EDSAC
• It was kicked around for a while with no
practical implementations
Copyright © 2005 – Curt Hill
History 2
• In the 1950s virtually every machine
had its own machine language
• Average lifetime of a machine was two
years
• Every two years every piece of
software had to be rewritten for the
new machine
• This was good motivation for both
microprogramming and high level
languages
Copyright © 2005 – Curt Hill
History 3
• In the February 1964 issue of
Datamation was an article decrying the
commercial failure of
microprogramming
• In April of the same year IBM
introduced the System 360
• All the low end machines were
microprogrammed
• The high end machines were
hardwired for performance
Copyright © 2005 – Curt Hill
Hardwired vs. Software
• A variety of applications can use either
a hard or soft solution
– Controllers being typical
• The hard solution is a digital gate
network
– Such as a Programmable Logic Array
• The soft solution is a microprocessor
with control program
• What are the tradeoffs?
Copyright © 2005 – Curt Hill
Tradeoffs
• Given the same level of technology the
hardwired solution is:
– Faster
– Uses fewer gates
• The software solution is:
– More flexible
– Less error prone
– Easier to maintain
• The Control Unit can be either
Copyright © 2005 – Curt Hill
The Control Unit
• May be hardwired with just a gate
network
• It may be microprogrammed as well
• Then known as a microcontroller
– Aka microprocesser, but that term is
ambiguous
• The software is then the microcode
• Microcode comes in two formats:
– Horizontal and vertical
Copyright © 2005 – Curt Hill
Microcode
• The goal of the microcode is to
sequence the ALU and other parts of
CPU to implement the instruction set
of the computer
• In this context the instruction set of the
processor is called the macrocode
• Generally the microcode is not visible
to the macrocode programmer
• The microcode only deals with very
low level features
Copyright © 2005 – Curt Hill
Vertical Microcode
• Vertical microcode is somewhat
similar to macrocode
• Each macrocode instruction becomes
a parameterless subroutine call
• The subroutine then implements the
macrocode instruction
• Consider the example of a 32 bit add
from System 360
Copyright © 2005 – Curt Hill
Add Example
• Consider the following add instruction
A r4,120(r6)
• It means this:
–
–
–
–
Add 120 to contents of register 6
Use that address to fetch a 32 bit item
Add that to contents of register 4
Leave the result in register 4
Copyright © 2005 – Curt Hill
CPU Units
ALU
ALU Result
Memory
Operand A
ALU Cmd
Operand B
Mem Cmd
Register
File
Copyright © 2005 – Curt Hill
Control
Unit
Vertical Microcode
Implementation
• Copy register 6 to ALU A
• Copy last 10 bits of instruction to ALU
B
• Execute ALU add
• Send result to memory as a fetch
• Send returned memory value to ALU A
• Send register 4 to ALU B
• Execute ALU add
• Send result to register 4
Copyright © 2005 – Curt Hill
Partial Implementation
• The above did nothing about several
important activities of the CPU
– Fetching the current instruction
– Determining the length of current
instruction
– Incrementing Program Counter
– Setting condition codes
• These may be part of the microcode
program or hardwired
– A control unit may have a mix of
hardwired and software implementation
Copyright © 2005 – Curt Hill
Problems
• Vertical microcode must do several
things for even a simple instruction
such as add
• Thus it must be very fast compared to
the basic hardware of the CPU
• It tends also to sequence the sub-parts
of the instruction
• This impairs performance since it does
not do well with parallel execution
Copyright © 2005 – Curt Hill
Vertical Microcode Example
• The LSI 11 used a vertical microcode
instruction
• Used a 22 bit micro instruction
• 11 bit micro address allowing 2048
words of micro instructions
• Somewhat similar to the PDP 11
instruction set that it emulated
• Supplied arithmetic, logical, general
and I/O operations
Copyright © 2005 – Curt Hill
Microcode Types
• Vertical microcode looks like
traditional assembly language
• The exception is the items that it deals
with are not always visible to the
assembly language programmer
• Thus it is easy for programmers to deal
with
• Horizontal microcode is a word where
each set of bits has a particular
meaning to the sub-units
Copyright © 2005 – Curt Hill
Horizontal Microcode
• Each instruction is a word divided into fields
• Each field is the command for one of the
units
• Using our example diagram our fields could
be:
– ALU
• Command
• Two Operands
• One result
– Memory
– Registers
Copyright © 2005 – Curt Hill
CPU Units
ALU
ALU Result
Memory
Operand A
ALU Cmd
Operand B
Mem Cmd
Register
File
Copyright © 2005 – Curt Hill
Control
Unit
ALU Operations
• Suppose our ALU has following commands:
–
–
–
–
–
–
–
–
Add
Not
And
Or
Shift left
Shift right
No operation
Continue
• Eight instructions – 3 bits of command
Copyright © 2005 – Curt Hill
Operands
• The ALU has three registers
• Two operand registers and one result
register
• There is an implied command for each
• For the two operand registers it is:
– Accept contents
– Do nothing
• For result it is:
– Output (to lines) result
– Do nothing
Copyright © 2005 – Curt Hill
Memory and Registers
• The registers and memory have three
commands:
– Fetch data
– Store data
– Do nothing
• In addition they both need an address
to get or fetch from
– Registers is typically 2-8 bits
• May be from microcode or instruction
– Memory is typically 32 or more bits
• Almost always computed
Copyright © 2005 – Curt Hill
Horizontal Microcode
0-2
ALU command
3
Operand A command
4
Operand B command
5
Result command
6-7
Memory command
8-9
Register command
Copyright © 2005 – Curt Hill
Microcode Word
• The above shows a 10 bit word
• Memory addresses are handled in other
• The microcode may have some bits to
handle choosing a register
• These words are dropped on the bus
periodically
• Hence two commands may execute at the
same time
– Storing a register value in memory could be
simultaneously done with an ALU computation
– The ALU continue command allows the ALU to
take more than one cycle to finish its computation
Copyright © 2005 – Curt Hill
Horizontal Microcode
• A different paradigm than
programmers expect
– Makes it more difficult to program
• The parallelism makes for faster
execution
Copyright © 2005 – Curt Hill
Example
• This time use horizontal micro code
– 10 opcode bits
• Consider the following add instruction
A r4,120(r6)
• It means this:
–
–
–
–
Add 120 to contents of register 6
Use that address to fetch a 32 bit item
Add that to contents of register 4
Leave the result in register 4
Copyright © 2005 – Curt Hill
Horizontal Microcode
• 000 1 0 0 00 01
– Copy register 6 to ALU A
• 000 0 1 0 00 01
– Copy last 10 bits of instruction to ALU B
• 001 0 0 0 00 00
– Execute ALU add
• 000 0 0 1 10 00
– Send result to memory as a fetch
• This ignores:
– How registers are selected
– How pieces of instruction are used
– All parallelism
Copyright © 2005 – Curt Hill
IBM 360 Example
• Microprogramming was used to make
all the machines to have the same
macroinstruction set
• The model 20 was a 16 bit machine
• The model 30, 40, 50, 60 were
microprogrammed 32 bit machine
• These came out before semiconductor
ROM
– Used IMPL to start
• The model 90 was a high end scientific
machine
Copyright © 2005 – Curt Hill
IBM 3033 Example
•
•
•
•
Successor to 360/370
Horizontal microcode
4096 word control store
Two sizes of micro instructions
– 2048 108 bit micro instructions
– 2048 126 bit micro instructions
Copyright © 2005 – Curt Hill
Partial Microprogramming
• The instruction set of a computer
could be mostly set
• That is the microcode is in ROM
• However, allow undefined opcodes to
be programmed as microcode
subroutines in some type of PROM
• IBM used unused opcodes in the 370
and 3000 series to speed the OS and
other products
– Common code made an instruction
Copyright © 2005 – Curt Hill
The Day Has Passed
• The heyday of microprogramming was the
60-70s
• It simplified CU design and made it easy
• Then CPUs were constructed out of discrete
chips
• The age of one chip microprocessors ended
the day
• Now a single CPU chip can generally run
software faster than the many chips with
custom microprogram
Copyright © 2005 – Curt Hill
Last Thoughts
• There is no reason why the
microprogram could not be a ROM
external to the main chip
• However, in most cases it is not
needed
• Only specialized CPUs such as runs
the Java Byte Code would need
• The price of software is too large for
most new machine designs
Copyright © 2005 – Curt Hill