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Advanced Computer Architecture-CS501
Advanced Computer Architecture
Lecture No. 16
Reading Material
Vincent P. Heuring & Harry F. Jordan
Chapter 4
Computer Systems Design and Architecture
4.2.2, 4.6.1
Control Signals Generation in SRC (continued...)
The Control Unit
2-Bus Implementation of the SRC Data Path
This section of lecture 16 is a continuation of the previous lecture.
Control signals for the store instruction
st ra, c2(rb)
The store time step operations are similar to the load instruction, with the exception of
steps T6 and T7. However, one can easily interpret these now. These are outlined in the
given table.
Control signals for the branch and branch link instructions
Branch instructions can be either be simple branches or link-and-then-branch type. The
syntax for the branch instructions is
brzr rb, rc
This is the branch and zero instruction we looked at earlier. The control signals for this
instruction are:
As usual, the first three steps are for the instruction fetch phase. Next, the following
control signals are issued:
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LCON to enable the CON circuitry to operate, and instruct it to check for the appropriate
condition (whether it is branch if zero, or branch if not equal to zero, etc.)
RCE to allow the register rc value to be read.
R2BUS allows the bus to read from the selected register.
At step T4:
RBE to allow the register rb value to be read. rb value is the branch target address.
R2BUS allows the bus to read from the selected register.
LPC (if CON=1): this control signal is issued conditionally, i.e. only if CON is 1, to
enable the write for the program counter. CON is set to 1 only if the specified condition is
met. In this way, if the condition is met, the program counter is set to the branch address.
Branch and link instructions
The branch and link instruction is similar to the branch instruction, with an additional
step, T4. Step T4 of the simple conditional branch instruction becomes the step T5 in this
The syntax of the instruction `branch and link if zero' is
brlzr ra, rb, rc
Table that lists the RTL and control signals for the store instruction of the SRC is given:
The circuitry that enables the condition checking for the conditional branches in the SRC
is illustrated in the following figure:
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Control signals for the shift right instruction
The given table illustrates the RTL and the control signals for the shift right `shr'
instruction. This is implemented by applying the five bits of n (nb4, nb3, nb2, nb1, nb0)
to the select inputs of the barrel shifter and activating the control signal SHR as explained
in an earlier lecture.
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Generating the Test Condition N=0
The Control Unit
The control unit is responsible for generating control signals as well as the timing signals.
Hence the control unit is responsible for the synchronization of internal as well as
external events. By means of the control signals, the control unit instructs the data path
what to do in every clock cycle during the execution of instructions.
Control Unit Design
Since the control unit performs quite complex tasks, its design must be done very
carefully. Most errors in processor design are in the Control Unit design phase. There are
primarily two approaches to design a control unit.
1. Hardwired approach
2. Micro programming
Hardwired approach is relatively faster, however, the final circuit is quite complex. The
micro-programmed implementation is usually slow, but it is much more flexible.
"Finite-state machine" concepts are usually used to represent the CU. Every state
corresponds to one "clock cycle" i.e., 1 state per clock. In other words each timing step
could be considered as just 1 state and therefore from one timing step to other timing
step, the state would change. Now, if we consider the control unit as a black box, then
there would be four sets of inputs to the control unit. These are as follows:
1. The output of timing step generator (There are 8 disjoint timing steps in our
example T0-T7).
2. Op-code (op-code is first given to the decoder and the output of the decoder is
given to the control unit).
3. Data path generated signals, like the "CON" control signal,
4. Signals from external events, like "Interrupt" generated by the Interrupt generator.
The complexity of the control is a function of the
 Number of states
 Number of inputs to the CU
 Number of the outputs generated by the CU
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Hardwired Implementation of the Control Unit
The accompanying block diagram shows the inputs to the control unit. The output control
signals generated from control unit to the various parts of the processor are also shown in
the figure.
Example Control Unit for the FALCON-A
The following figure shows how the operation code (op-code) field of the Instruction
Register is decoded to generate a set of signals for the Control unit.
This is an example for the FALCON-A processor where the instruction is 16-bit long.
Similar concepts will apply to the SRC, in which case the instruction word is 32 bits and
IR <31...27> contains the op-code. Similar concepts will apply to the SRC, in which case
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the instruction word is 32 bits and IR<31..27> contains the opcode. The most significant
5 bits represent the op-code. These 5-bits from the IR are fed to a 5-to-32 decoder. These
32 outputs are numbered from 0-to-31 and named as op0, op1 up to op31. Only one of
these 32 outputs will be active at a given time .The active output will correspond to
instruction executing on the processor.
To design a control unit, the next step is to write the Boolean Equations. For this we need
to browse through the structural descriptions to see which particular control signals occur
in different timing steps. So, for each instruction we have one such table defining
structural RTL and the control signals generated at each timing step. After browsing we
need to check that which control signal is activated under which condition. Finally we
need to write the expression in the form of a logical expression as the logical combination
of "AND" and "OR" of different control signals. The given table shows Boolean
Equations for some example control signals.
For example, PCout would be active in every T0 timing step. Then in timing interval T3
the output of the PC would be activated if the op-code is 20 or 22 which represent jump
and sub-routine call. In step T4 if the op-code is 16, 17, 18 or 19, again we need PCout
activated and these 4 instructions correspond to the conditional jumps. We can say that in
other words in step T1, PCout is always activated "OR" in T3 it is activated if the
instruction is either jump or sub-routine call "OR" in T4 if there is one of the conditional
jumps. We can write an equation for it as
In the form of logic circuit the implementation is shown in the figure. We can see that we
"OR" the op-ode 20 and 22 and "AND" it with T3, then "OR" all the op16 up to op19
and "AND" it with T4, then T0 and the "AND" outputs of T3 and T4 are "OR" together
to obtain the PCout.
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In the same way the logic circuit for LPC control signal is as shown and the equation
would be :
We can formulate Boolean equations and draw logic circuits for other control signals in
the same way.
Effect of using "real" Gates
We have assumed so far that the gates are ideal and that there is no propagation delay. In
designing the control unit, the propagation delays for the gates can not be neglected. In
particular, if different gates are cascaded, the output of one gate forms the input of other.
The propagation delays would add up. This, in turn would place an upper limit on the
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frequency of the clock which controls the generation of the timing intervals T0, T1... T7.
So, we can not arbitrarily increase the frequency of this clock. As an example consider
the transfer of the contents of a register R1 to a register R2. The minimum time required
to perform this transfer is given by
tmin = tg + tbp + tcomb + t1
The details are explained in the text with reference to Fig 4.10. Thus, the maximum clock
frequency based on this transfer will be 1/tmin. Students are encouraged to study example
4.1 of the text.
2-Bus Implementation of the SRC Data Path
In the previous sections, we studied the uni-bus implementation of the data path in the
SRC. Now we present a 2-bus implementation of the data path in the SRC. We observe
from this figure that there is a bus provided for data that is to be written to a component.
This bus is named the `in' bus. Another bus is provided for reading out the values from
these components. It is called the `out' bus.
Structural RTL for the `sub' instruction using the 2-bus data path implementation
Next, we look at the structural RTL as well as the control signals that are issued in
sequence for instruction execution in a 2-bus implementation of the data path. The given
table illustrates the Register Transfer Language representation of the operations for
carrying out instruction fetch, and execution for the sub instruction.
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The first three steps belong to the instruction fetch phase; the instruction to be executed is
fetched into the Instruction Register and the PC value is incremented to point to the next-
in-line instruction. At step T3, the register R[rb] value is written to register A. At the time
step T4, the subtracted result from the ALSU is assigned to the destination register R[ra].
Notice that we did not need to store the result in a temporary register due to the
availability of two buses in place of one. At the end of this sequence, the timing step
generator is initialized to T0.
Control signals for the fetch operation
The control signals for the instruction fetch phase are shown in the table. A brief
explanation is given below:
At time step T0, the following control signals are issued:
 PCout: This will enable read of the Program Counter, and so its value will be
transferred onto the `out' bus
 LMAR: To enable the load for MAR
 C=B: This instruction is used to copy the value on the `out' bus to the `in' bus, so
it can be loaded into the Memory Address Register. We can observe in the data-
path implementation figure given earlier that, at any time, the value on the `out'
bus makes up the operand B for the ALSU. The result C of ALSU is connected to
the "in" bus, and therefore, the contents transfer from one bus to the other can
take place.
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At time step T1:
 PCout: Again, this will enable read of the Program Counter, and so its value will
be transferred onto the CPU internal `out' bus
 INC4: To instruct the ALSU to perform the increment-by-four operation.
 LPC: This control signal will enable write of the Program Counter, thus the new,
incremented value can be written into the PC if it is made available on the "in"
bus. Note that the ALSU is assumed to include an INC4 function.
 MRead: To enable memory word read.
 MARout: To supply the address of memory word to be accessed by allowing the
contents of the MAR (memory address register) to be written onto the CPU
external (address) bus.
 LMBR: The memory word is stored in the register MBR (memory buffer
register) by applying this control signal to enable the write of the MBR.
At time step T2:
 MBRout: The contents of the Memory Buffer Register are read out onto the
`out' bus, by means of applying this signal, as it enables the read for the MBR.
 C=B: Once again, this signal is used to copy the value from the `out' bus to the
`in' bus, so it can be loaded into the Memory Address Register.
 LIR: This instruction will enable the write of the Instruction Register. Hence the
instruction that is on the `in' bus is loaded into this register.
At time step T3, the execution may begin, and the control signals issued at this stage
depend on the actual instruction encountered. The control signals issued for the
instruction fetch phase are the same for all the instructions.
Note that, we assume the memory to be fast enough to respond during a given time slot.
If that is not true, wait states have to be inserted. Also keep in mind that the control
signals during each time slot are activated simultaneously, while those for successive
time slots are activated in sequence. If a particular control signal is not shown, its value is
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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