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Instruction Set
Architecture
CS2052 Computer Architecture
Computer Science & Engineering
University of Moratuwa
Dilum [email protected]
Blocks of a Microprocessor
2
Literal
Address
Operation
Program
Memory
Instruction
Register
STACK Program CounterInstruction
Decoder
Timing, Control and Register selection
Accumulator
RAM &
Data
Registers
ALU
IO
IOFLAG &
Special
Function
Registers
Clock
Reset
Interrupts
Program Execution Section Register Processing Section
Set upSet up
Modify
Address
Internal data bus
Source: Makis Malliris & Sabir Ghauri, UWE
Instruction Set Architecture (ISA)
3
Instruction Set
Software
Hardware
Source: Computer Architecture: A Quantitative Approach, J. L. Hennessy & D. A. Patterson, 3rd Edition.
ISA (Cont.)
Part of computer architecture related to
programming
Include native data types, instructions, registers,
addressing modes, memory architecture,
interrupt & exception handling, & external I/O
e.g., R1, R2, …, PC
e.g., MOV, ADD, INC, AND
ISA specifies the set of opcodes (machine
language), & native commands implemented by
a particular processor
4
Well Known ISAs
x86
Based on Intel 8086 CPU in 1978
Intel family, also followed by AMD
X86-64
64-bit extensions
Proposed by AMD, also followed by Intel
ARM
32-bit & 64-bit
Initially by Acorn RISC Machine
ARM Holding
MIPS
32-bit & 64-bit
By Microprocessor without Interlocked Pipeline Stages (MIPS)
Technologies 5
Well Known ISAs (Cont.)
SPARC
32-bit & 64-bit
By Sun Microsystems
PIC
8-bit to 32-bit
By Microchip
Z80
8-bit
By Zilog in 1976
Many extensions
Intel – MMX, SSE, SSE2, AVX
AMD – 3D Now!
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A Good ISA
Lasts through many implementations
Portability, compatibility
Used in many different ways
Servers, desktop, laptop, mobile, tablet
Provides convenient functions to higher layer
Permit efficient implementation at lower layer
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Example – Instructions
Microprocessor that we are going to build will
support following 2 instructions
ADD
LOAD
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Activating Necessary Blocks
9Source: www.transtutors.com/homework-help/computer-
science/computer-architecture/cpu/general-register-organization/
Micro-operations
Digital modules are best described by
Registers they contain
Micro-operations performed on data stored on those
registers
Elementary operations done on data in registers
Register Transfer Language
Concise symbolic expressions
ADD ADD RC, RA, RB RC RA + RB
PC PC + 1
LOAD LOAD RA, d RA d
PC PC + 110
How to Describe a Computer
Set of registers & their functions
Sequence of micro-operations
Controls that initiates & maintains sequence of
micro-operations
Today, from a programming point of view,
Assembly is the lowest level we use to define
these registers, instructions, & their order of
execution
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Programming in Assembly
To program in Assembly we need
1. Knowledge about hardware design
Registers
Memory addressing
I/O
2. Knowledge about instruction set
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Registers (Review)
Type of memory located inside CPU
Can hold a single piece of data
Useful in both data processing & control
functionalities
Types
Special purpose registers
Program counter (PC)
Instruction register (IR)
Accumulator or working register (A or W)
Flag register (FLAG or STATUS)
General purpose registers
No special name, typically A, B, C, ... Or R1, R2, R3, ... 13
Format of an Assembly Statement
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[Identifier /
Label]
Operation/
Command/
Op code
[Operand(s)] [;Comment]
Labeling code or
to indicate a
program
destination
address for
jumps
What instruction to
be carried out by
CPU
Only valid
instructions are
allowed
Data or register
contents to be used
in instruction
Some instructions
don’t need operands
Explanatory text.
Optional but very
useful, as
Assembly
programs are
hard to
understand
L20: ADD RC, RA, RB ;RC RA + RB
Example – Instruction Set
We’ll use instruction set from PIC 16F87x for our
discussion
Textbook doesn’t use a specific set
Most other textbooks may use MIPS or x86
They are still too complex to start with
When you are more familiar, you can learn/use any
new instruction set
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16
F file register
W working register
B bit
L literal (number)
Z conditional
execution
d destination bit
d=0 store in W
d=1 store in f
use , w or ,f instead
Source: Makis Malliris &
Sabir Ghauri, UWE
Opcode
Determines the
instruction
Registers, bits,
literals depend on
the opcode field
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Source: PIC16F87X Data Sheet by
Microchip Technology Inc.
Assembler
Assembler translates human readable code into
binary
Binary code specifies opcode & operands
ADDLW 135 means “add literal 135 to W register”
Assembler converts this to 11 1110 1000 0111
This is what the machine understands
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Program Operations
Load a register with a given number
Copy data from register to another
Carry out arithmetic & logical operations on a
data word or a pair of data words
Test a bit or word & jump, or not, depending on
result of the test
Jump to a point in the program
Jump to a subroutine
Carry out a special control operation
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Instruction Classification
Instruction Types
Data transfers
Arithmetic, logic
Rotates, shifts
Bit set, bit reset, & bit test
General-purpose, CPU
control
Jumps
Calls, returns
Instruction Function
Move
Register
Arithmetic
Logic
Test & Skip
Jump
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Data Transfer Instructions
Used to transfer data from one location to
another
Register to Register, Register to Memory, Memory to
Register
MOV
MOVLW 3 ; W 03h
MOVWF R1 ; R1 03h
MOV is same as LDA (Load) in textbook
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Arithmetic Instructions
Used in arithmetic operations
ADD – ADDWF R1 ; W W + R1
ADD – ADDLW 3 ; W W + 3
SUB – SUBLW 5 ; W W - 5
INC – INCF R1 ; R1 R1 + 1
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ALU (Review)
Data processing
unit
Arithmetic unit
Performs
arithmetic
operations
Logic unit
Performs logical
operations
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Accumulator
Source: Introduction to PIC Microcontroller – Part 1 by Khan Wahid
Example – Arithmetic Instructions
Write an assembly program to add 5 & 10
Steps
How many registers?
What registers to use?
What instructions are required?
MOV – MOVLW 5 ; W 05h
ADD – ADDLW 0xA ; W 05h + Ah
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Logic Operations
Used in bitwise logic operations
AND – ANDLW 3 ; W W & 0011
OR – IORLW 3 ; W W | 0011
XOR – XORLW 3 ; W W 1001
NOT – COMF 0x20,1 ;(0x20) (0x20)/
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Example – Logic Operations
Example
Write an assembly program to convert a given
character from uppercase to lowercase & vice versa
If we consider ASCII, this can be achieved by
changing the 5th bit
A = 65 =0x41 = 0 1 0 0 0 0 0 1
a = 97 =0x61 = 0 1 1 0 0 0 0 1
Get XOR with 00100000 = 32 = 0x20
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Transfer Instructions (Jump
Instructions)
Can be used to jump here & there within program
Can be used to control loops
GOTO – GOTO loop ;go to loop label
CALL – CALL delay ;call delay subroutine
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Example – Transfer Instructions
Example Write a program to calculate the total of all integers
from 1 to 10
High-level program
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int total = 0;
for (int i=1 ; i<=10; i++)
{
total += i;
}
Example – Transfer Instructions
(Cont.)
Steps
Are there any conditions/loops?
How many registers?
What registers to use?
What instructions to use?
ADDLW
Increment/decrement instruction – INCF, DECF
Let’s use memory address 0020h
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Transfer Instructions (Cont.)
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start movlw 0xa ; w 10
movwf 0x20 ; (0x20) w
movlw 0 ; total
loop addwf 0x0020,0 ; Add
decf 0x0020,1 ; Dec counter
btfss STATUS,Z ; is counter 0?
goto loop ; repeat
nop
end
int total = 0;
for (int i=10 ; i!=0; i--)
{
total += i;
}
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Homework
Example
Write an assembly program to multiply 3 & 4
Steps:
How many registers?
What registers to use?
What instructions to use?
MOV – MOVLW 3 ; W 03h
MUL – ???
Shift Operators >> <<
Move all bits in a value by given no of positions to left or
right10011011 01110110 10010011
>> 1 >> 3 >> 3
01001101 00001110 00010010
10011011 01110110
<<1 <<3
00110110 10110000
Multiply by powers of 2 – value << n (value * 2n)
e.g., 4 << 3 ; (4 * 8) =32
Divide by powers of 2 – value >>=n (value / 2n)
e.g., 75 >> 4; 75 / 16 =4
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Rotate Through Carry
RLF – Rotate Left f through Carry
Suppose carry was 1
RLF 9B 1 = 37
RLF 9B 2 = 6F
RRF – Rotate Right f through Carry
Suppose carry was 1
RRF 9B 1 = CD
RRF 9B 2 = E6 33
Comparison Operations
Comparing 2 numbers need to be treated with
care due to non-intuitive nature of Carry flagmovlw 2 ; W = 2
subwf Q, W ; W = Q - 2
btfss STATUS, C ; check carry flag
goto Gr_eq ; Q >= 2
goto Less ; Q < 2
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