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Advanced Computer Architecture-CS501
Advanced Computer Architecture
Lecture No. 22
Reading Material
Vincent P. Heuring&Harry F. Jordan
Chapter 5
Computer Systems Design and Architecture
Working of a General Microcoded Controller
Microprogram Memory
Generating Microcode for Some Sample Instructions
Horizontal and Vertical Microcode Schemes
Microcoded 1-bus SRC Design
The SRC Microcontroller
In the previous lectures, we have discussed how to implement logic circuitry for a control
unit based on logic gates. Such an implementation is called a hardwired control unit. In a
micro programmed control unit, control signals which need to be generated at a certain
time are stored together in a control word. This control word is called a microinstruction.
A collection of microinstructions is called a microprogram. These microprograms
generate the sequence of necessary control signals required to process an instruction.
These microprograms are stored in a memory called the control store.
As described above microprogramming or microcoding is an alternative way to design
the control unit. The microcoded control unit is itself a small stored program computer
consisting of
Microprogram memory
Microinstruction word
Comparison of hardwired and microcoded control unit
Hardwired Control Unit
Microcoded Control Unit
The relationship between control The control signals here are stored as words
inputs and control outputs is a series in a microcode memory.
of Boolean functions.
Hardwired control units are generally Microcode units simplify the computer logic
but it is comparatively slower.
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Working of a general microcoded controller
A microcoded controller works in the same way as a small general purpose computer.
1. Fetch a micro-instruction and increment micro-PC.
2. Execute the instruction present in micro-IR.
3. Fetch the next instruction and so on...
The microcoded control unit is like
a small computer in itself. It
memory, which is read using a
micro program counter. The micro
microprogram controller. Values of
the micro PC depends on a 4 to 1
MUX. The source may be the
incremented micro PC value, or a
calculated branch value, or a value
derived by decoding an opcode for
an instruction. The microprogram
memory writes the control word at
the chosen address into the micro
instruction register. This control word is basically the set of all the control signals needed
to execute the instruction at that particular instant.
Fields in the micro instruction
C Bits
These form the control signal
M Bits
These form the branch address
B Bits
These form the branch control
Loading the micro-PC
The micro-PC can be loaded from one of the four possible sources
Simple increment Steps sequentially from microinstruction to microinstruction
Lookup table A lookup table maps the opcode field to the starting address of the
microcode routine that generates control signals.
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External source Initializes micro-PC to begin an operation e.g. interrupts service, reset
Branch addresses Jumps anywhere in the microprogram memory on the basis of
conditional or unconditional branch.
Microprogram Memory
This small memory contains microroutines for all the instructions in the ISA
The micro-PC supplies the address and it returns the control word stored at that
It is much faster and smaller than a typical main memory
Layout of a typical microprogram memory
Generating Microcode for Some Sample Instructions
The control word for an instruction is used to generate the equivalent microcode
Each step in RTL corresponds to a microinstruction executed to generate the control
Each bit in the control words in the microprogram memory represents a control signal.
The value of that bit decides whether the signal is to be activated or not.
Example: Control Signals for the sub Instruction
The first three addresses from 100 to 102 represent microcode for instruction fetch and
the last three addresses from 203 to 205 represent microcode for sub instruction. In the
first cycle at address 100, the control signal PCout, LMAR, LC, and INC4 are activated
and all other signals are deactivated. All these control signals are for the SRC processor.
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So, if the micro-PC contains 100, the contents of microprogram memory are copied into
the micro IR. This corresponds to the structural RTL description of the T0 clock during
instruction fetch phase. In the same way, the content of address 101 corresponds to T1,
and the content of address 102 corresponds to T2.
Microprogram Controller functions: Branching and looping
Microprogram controller
controls the sequence of
The  inputs  to  the
microcontroller are from
the branch control fields
specified in the microcode
Its output controls the 4
to 1 multiplexer inside the
microcoded control unit.
conditional execution and
unconditional branch
If a branch instruction is encountered within the microprogram hardwired logic selects
the branch address as the source of micro-PC using 4 to 1 mux. This hardwired logic
caters for all branch instructions including branch if zero.
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4-1 Multiplexer
The multiplexer supplies one of the four possible values to the micro-PC
The incremented value of the micro-PC is used when dealing with the normal flow of
The opcode from the instruction is used to set the micro-PC when a microroutine is
initially being loaded.
External address is used when it is required to reset the microprogram controller.
Branch address is set into the micro-PC when a branch microinstruction is encountered.
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Advanced Computer Architecture
Lecture 22
How to form a branch
A branch can be implemented by choosing one alternative from each of the following
two lists.
This scheme provides flexibility in choosing branches as we can form any combination
of conditions and addresses.
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Microcode Branching Examples
Following is an example of branch instructions in microcode
400 00 0 0 0
xxx No  branch,goto  next {...};
address in sequence-401
401 01 1 0 0
xxx To the address supplied {...};
by opcode
initial address;
402 10 0 0 1
xxx To external address if Z {...}; if Z then
flag is set
goto Ext. Add.
403 11 0 0 0
200 To 200 if N flag is set, {...}; if N then
else to 404
goto Label1;
404 11 0 0 0
1 0 000
406 To 406 if N is false, else While
to 405
405 11 1 0 0
404 Branch to 404
While contd...
Similarity between microcode and high level programs
Any high level construct such as if-else, while, repeat etc. can be implemented using
A variety of microcode compilers similar to the high level compilers are available that
allow easier programming in microcode
This similarity between high level language and microcode simplifies the task of
controller design.
Horizontal and vertical microcode schemes
In horizontal microcode schemes, there are no intermediate decoders and the control
word bits are directly connected to their destination i.e. each bit in the control word is
directly connected to some control signal and the total number of bits in the control word
is equal to the total number of control signals in the CPU.
Vertical microcode schemes employ an extra level of decoding to reduce the control
word width. From an n bit control word we may have 2n bit signal values.
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However, a completely vertical scheme is not feasible because of the high degree of fan
Horizontal Microcode Scheme
Vertical Microcode Scheme
Microcoded 1-bus SRC design
In the SRC the bits from the opcode in the instruction register are decoded to fetch the
address of the suitable microroutine from the microprogram memory. The microprogram
controller for the SRC microcoded control unit employs the logic for handling exceptions
and reset process. Since the SRC does not have any condition codes, we use the CON and
n signals instead of N and Z flags to control branches in case of branch if equal to zero or
branch if less than instructions.
The SRC Microprogram Controller
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The microprogram controller for the SRC microcoded control unit employs the logic
for handling exceptions and reset process
Since the SRC does not have any condition codes, we use the CON and n signals
instead of N and Z flags to control branches
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Microcode for some SRC instructions
300 00
xxx MAR
PC + 4;
301 00
xxx MBR
xxx IR,Micro-PC MBR<31...27>;
302 01
400 00
xxx A
401 00
xxx C
A + R[rc];
402 11
300 R[ra]
C; Micro-PC
Assume the first control word at address 300. The RTL of this instruction is MAR
combined with C  PC+4. To facilitate these actions the PCout signal bit and the LMAR
signal bit are set to one, so that the value of the PC may be written to the internal
processor bus and written onto the MAR. The instructions at 300, 301 and 302 form the
microcode for instructions fetch. If we examine the RTL we can see all the functionality
of the fetch instruction. The value of PC is incremented, the old value of PC is sent to
memory, the instruction from the sent address is loaded into memory buffer register.
Then the opcode of the fetched instruction is used to invoke the appropriate microroutine.
Alternative approaches to microcoding
Bit ORing
Writable Microprogram Memory
Subroutines in Microprogramming
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Table of Contents:
  1. Computer Architecture, Organization and Design
  2. Foundations of Computer Architecture, RISC and CISC
  3. Measures of Performance SRC Features and Instruction Formats
  4. ISA, Instruction Formats, Coding and Hand Assembly
  5. Reverse Assembly, SRC in the form of RTL
  6. RTL to Describe the SRC, Register Transfer using Digital Logic Circuits
  7. Thinking Process for ISA Design
  8. Introduction to the ISA of the FALCON-A and Examples
  9. Behavioral Register Transfer Language for FALCON-A, The EAGLE
  10. The FALCON-E, Instruction Set Architecture Comparison
  11. CISC microprocessor:The Motorola MC68000, RISC Architecture:The SPARC
  12. Design Process, Uni-Bus implementation for the SRC, Structural RTL for the SRC instructions
  13. Structural RTL Description of the SRC and FALCON-A
  14. External FALCON-A CPU Interface
  15. Logic Design for the Uni-bus SRC, Control Signals Generation in SRC
  16. Control Unit, 2-Bus Implementation of the SRC Data Path
  17. 3-bus implementation for the SRC, Machine Exceptions, Reset
  18. SRC Exception Processing Mechanism, Pipelining, Pipeline Design
  19. Adapting SRC instructions for Pipelined, Control Signals
  20. SRC, RTL, Data Dependence Distance, Forwarding, Compiler Solution to Hazards
  21. Data Forwarding Hardware, Superscalar, VLIW Architecture
  22. Microprogramming, General Microcoded Controller, Horizontal and Vertical Schemes
  23. I/O Subsystems, Components, Memory Mapped vs Isolated, Serial and Parallel Transfers
  24. Designing Parallel Input Output Ports, SAD, NUXI, Address Decoder , Delay Interval
  25. Designing a Parallel Input Port, Memory Mapped Input Output Ports, wrap around, Data Bus Multiplexing
  26. Programmed Input Output for FALCON-A and SRC
  27. Programmed Input Output Driver for SRC, Input Output
  28. Comparison of Interrupt driven Input Output and Polling
  29. Preparing source files for FALSIM, FALCON-A assembly language techniques
  30. Nested Interrupts, Interrupt Mask, DMA
  31. Direct Memory Access - DMA
  32. Semiconductor Memory vs Hard Disk, Mechanical Delays and Flash Memory
  33. Hard Drive Technologies
  34. Arithmetic Logic Shift Unit - ALSU, Radix Conversion, Fixed Point Numbers
  35. Overflow, Implementations of the adder, Unsigned and Signed Multiplication
  36. NxN Crossbar Design for Barrel Rotator, IEEE Floating-Point, Addition, Subtraction, Multiplication, Division
  37. CPU to Memory Interface, Static RAM, One two Dimensional Memory Cells, Matrix and Tree Decoders
  38. Memory Modules, Read Only Memory, ROM, Cache
  39. Cache Organization and Functions, Cache Controller Logic, Cache Strategies
  40. Virtual Memory Organization
  41. DRAM, Pipelining, Pre-charging and Parallelism, Hit Rate and Miss Rate, Access Time, Cache
  42. Performance of I/O Subsystems, Server Utilization, Asynchronous I/O and operating system
  43. Difference between distributed computing and computer networks
  44. Physical Media, Shared Medium, Switched Medium, Network Topologies, Seven-layer OSI Model