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Advanced Computer Architecture-CS501
Advanced Computer Architecture
Lecture No. 13
Reading Material
Vincent P. Heuring & Harry F. Jordan
Chapter 4
Computer Systems Design and Architecture
4.2.2, slides
Structural RTL Description of the SRC (continued...)
Structural RTL Description of the FALCON-A
This lecture is a continuation of the previous lecture.
Structural RTL for branch instructions
Let us take a look at the structural RTL for branch instructions. We know that there are
several variations of the branch instructions including unconditional branch and different
conditional branches. We look at the RTL for `branch if zero' (brzr) and `branch and link
if zero' brlzr' conditional branches.
The syntax for the branch if zero (brzr) is:
brzr rb, rc
As you may recall, this instruction
instructs the processor to branch to the
instruction at the address held in
register rb, if the value stored in
register rc is zero. Time steps for this
instruction are outlined in the table.
The first three steps are of the
instruction fetch phase. Next, the value
of register rc is checked and depending
on the result, the condition flag CON is set. In time step T4, the program counter is set to
the register rb value, depending on the CON bit (the condition flag).
The syntax for the branch and link if zero (brlzr) is:
brlzr ra, rb, rc
This instruction is the same as the
instruction brzr but additionally the
return  address  is  saved  (linking
procedure). The time steps for this
instruction are shown in the table.
Notice  that  the  steps  for  this
instruction are the same as the
instruction brzr with an additional step
after the condition bit is set; the current
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value of the program counter is saved to register ra.
Structural RTL for shift instructions
complicated in the sense that they
require extra hardware to hold and
decrement the count. For an ALSU
that can perform only single bit shifts,
the data must be repeatedly cycled
through the ALSU and the count
decremented until it reaches zero. This
approach  presents  some  timing
problems, which can be overcome by
employing multiple-bit shifts using a
barrel shifter.
The structural RTL for shr ra, rb, rc or shr ra, rb, c3 is given in the corresponding
table shown. Here n represents a 5-bit register; IR bits 0 to 4 are copied in to it. N is the
decimal value of the number in this register. The actual shifting is being done in step T5.
Other instructions that will have similar tables are: shl, shc, shra
e.g., for shra, T5 will have C(NαR [rb] <31>) R[rb] <31...N>;
Structural RTL Description of FALCON-A Instructions
Uni-bus data path implementation
Comparing the uni-bus implementation of FALCON-A with that of SRC results in the
following differences:
ˇ  FALCON-A processor bus has 16 lines or is 16-bits wide while that of SRC is
32-bits wide.
ˇ  All registers of FALCON-A are of 16-bits while in case of SRC all registers are
ˇ  Number of registers in FALCON-A are 8 while in SRC the number of registers is
ˇ  Special registers i.e. Program Counter (PC) and Instruction Register (IR) are 16-
bit registers while
in SRC these are
ˇ  Memory  Address
and Memory Buffer
Register (MBR) are
also  of  16-bits
while in SRC these
are of 32-bits.
MAR and MBR are dual
port registers. At one side
they  are  connected  to
internal bus and at other
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side to external memory in order to point to a particular address for reading or writing
data from or to the memory and MBR would get the data from the memory.
ALSU functions needed
ALSU of FALCON-A has slightly different functions. These functions are given in the
Note that mul and div
are two significant
instructions in this
instruction set. So
whenever one of these
instructions is activated,
the ALSU unit would
take the operand from
its input and provide the
output immediately, if
we neglect the
propagation delays to
its output. In case of
FACON-A, we have
two registers A and AH
each of 16-bits. AH
would contain the
higher 16-bits or most significant 16-bits of a 32-bit operand. This means that the ALSU
provides the facility of using 32-bit operand in certain instructions. At the output of
ALSU we could have a 32-bit result and that can not be saved in just one register C so we
need to have another one that is CH. CH can store the most significant 16-bits of the
Why do we need to add AH and CH?
This is because we have mul and div instructions in the instruction set of the FALCON-
A. So for that case, we can implement the div instruction in which, at the input, one of the
operand which is dividend would be 32-bits or in case of mul instruction the output
which is the result of multiplication of two 16-bit numbers, would be 32-bit that could be
placed in C and CH. The data in these 2 registers will be concatenated and so would be
the input operand in two registers AH and A. Conceptually one could consider the A and
AH together to represent 32-bit operand.
Structural RTL for subtract
sub ra, rb, rc
In sub instruction three registers are
involved. The first three steps will
fetch the sub instruction and in T3,
T4, T5 the steps for execution of
the sub instruction will be
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Structural RTL for addition
add ra, rb, rc
The table of add instruction is
almost same as of sub instruction
except in timing step T4 we have +
sign for addition instead of ­ sign
as in sub instruction. Other instructions that belong to the same group are `and', `or' and
Structural RTL for multiplication instruction
mul ra, rb, rc
This instruction is only present in this processor and not in SRC. The first three steps are
exactly same as of other
instructions and would fetch the
mul instruction. In step T3 we will
bring the contents of register R [rb]
in the buffer register A at the input
of ALSU. In step T4 we take the
multiplication of A with the contents of R[rc] and put it at the output of the ALSU in two
registers C and CH. CH would contain the higher 16-bits while register C would contain
the lower 16-bits. Now these two registers cannot transfer the data in one bus cycle to the
registers, since the width is 16-bits. So we need to have 2 timing steps, in T5 we transfer
the higher byte to register R[0] and in T6 the lower 16-bits are transferred to the
placeholder R[a]. As a result of multiplication instruction we need 3 timing steps for
Instruction Fetch and 4 timing steps for Instruction Execution and 7 steps altogether.
Structural RTL for division instruction
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div ra, rb, rc
In this instruction first three steps
are the same. In step T3 the
contents of register rb are placed in
buffer register A and in step T4 we
take the contents of register R[0] in
to the register AH. We assume
before using the divide instruction that we will place the higher 16-bits of dividend to
register R[0]. Now in T5 the actual division takes place in two concurrent operations. We
have the dividend at the input of ALSU unit represented by concatenation of AH and A.
Now as a result of division instruction, the first operation would take the remainder. This
means divide AH concatenated with A with the contents given in register rc and the
remainder is placed in register CH at the output of ALSU. The quotient is placed in C. In
T6 we take C to the register R[ra] and in T7 remainder available in CH is taken to the
default register R[0] through the bus. In divide instruction 5 timing steps are required to
execute the instruction while 3 to fetch the instruction.
Note: Corresponding to mul and div instruction one should be careful about the
additional register R[0] that it should be properly loaded prior to use the instructions e.g.
if in the divide instruction we don't have the appropriate data available in R[0] the result
of divide instruction would be wrong.
Structural RTL for not instruction
not ra, rb
In this instruction first three steps
will fetch the instruction. In T3 we
perform  the  not  operation  of
contents in R[rb] and transfer them
in to the buffer register C. It is
simply  the  one's  complement
changing of 0's to 1's and 1's to
0's. In timing step T4 we take the
contents of register C and transfer to register R[ra] through the bus as shown in its
corresponding table.
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Structural RTL for add immediate instruction
addi ra, rb, c1
In this instruction c1 is a constant as a part of the instrucion. First three steps are for
Instruction Fetch operation. In T3
we take the contents of register R
[rb] in to the buffer register A. In
T4 we add up the contents of A
with the constant c1 after sign
extension and bring it to C.
Sign extension of 5-bit c1 and
8-bit constant c2
Sign extension for 5-bit c1 is: (11αIR<4> ŠIR<4.. 0>)
We have immediate constant c1 in the form of lower 5-bits and bit number 4 indicates the
sign bit. We just copy it to the left most 11 positions to make it a 16-bit number.
Sign extension for 8-bit c2 is:
(8αIR<7> ŠIR<7.. 0>)
In the same way for constant c2 we need to place the sign bit to the left most 8 position to
make it 16-bit number.
Structural RTL for the load
and store instruction
Tables for load and store
instructions are same as
SRC  except  a  slight
difference in the notation.
So when we have square
brackets [R [rb]+c1], it
corresponds to the base
address in R[rb] and an offset taken from c1.
Structural RTL for conditional jump
jz ra, [c2]
In first three steps of this table, the
instruction is fetched. In T3 we set a 1-
bit register "CON" to true if the
condition is met.
How do we test the condition?
This is tested by the contents given by
the register ra. So condition within
square brackets is R[ra]. This means
test the data given in register ra. There
are different possibilities and so the data could be positive, negative or zero. For this
particular instruction it would be tested if the data were zero. If the data were zero, the
"CON" would be 1.
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In T4 we just take the contents of the PC into the buffer register A. In T5 we add up the
contents of A to the constant c2 after sign extension. This addition will give us the
effective address to which a jump would be taken. In T6, this value is copied to the PC.
In FALCON-A, the number of conditional jumps is more than in SRC. Some of which
are shown below:
ˇ  jz (op-code= 19) jump if zero
jz r3, [4]
(R[3]=0): PCPC+ 2;
ˇ  jnz (op-code= 18) jump if not zero
jnz r4, [variable]
(R[4]0): PCPC+ variable;
ˇ  jpl (op-code= 16) jump if positive
jpl r3, [label]
(R[3]0): PC PC+ (label-PC);
ˇ  jmi (op-code= 17) jump if negative
jmi r7, [address]
(R[7]<0): PCPC+ address;
The unconditional jump instruction will be explained in the next lecture.
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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