COSC 3P92
Cosc 3P92
Week 4 Lecture slides
It is the mark of an educated mind to be able to entertain
a thought without accepting it.
Aristotle
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COSC 3P92
Computer Instruction Set
• An instruction has two components:
– e.g
.
Op-code
ADD
Operand(s)
R0
100
• The operand field may have the following
formats:
1)
2)
3)
4)
zero-address
one-address
two-address
three-address
• The total number of instructions and the types
and formats of the operands determine the
length of an instruction.
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COSC 3P92
Computer Instruction Set
• The shorter the instruction, the faster the time
that it can be fetched and decoded.
• Shorter instructions are better than longer ones:
(i) take up less space in memory
(ii) transferred to the CPU faster
• A machine with 2^N instructions must require at
least N-bit to encode all the op-codes.
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COSC 3P92
Bits/cell (word)
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COSC 3P92
Instruction sets
• Byte ordering
– Big-endian: bytes in word ordered from left-to-right eg.
Motorola
– Little-endian: bytes in word ordered right-to-left eg. Intel
• Creates havoc when transferring data; need to
swap byte order in transferred words.
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COSC 3P92
Big/Little Endian
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COSC 3P92
Op-code Encoding
• 1. Block-code technique
– To each of the 2K instructions a unique binary bit
pattern of length K is assigned.
– An K-to-2K decoder can then be used to decode all
the instructions. For example,
3-bit Op-code
3-to-8
decoder
instruction 0
instruction 1
instruction 2
instruction 3
instruction 4
instruction 5
instruction 6
instruction 7
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COSC 3P92
Op-code Encoding
• 2. Expanding op-code technique
– Consider an 4+12 bit instruction with a 4-bit op-code and three 4-bit
addresses.
Op-code
Address 1
Address 2
Address 3
– It can at most encode 16 three-address instructions.
– If there are only 15 such three-address instructions, then one of the
unused op-code can be used to expand to two-address, one-address
or zero address instructions
1111
Op-code
Address 1
Address 2
– Again, this expanded op-code can encode at most 16 two-address
instructions. And if there are less than 16 such instructions, we can
expand the op-code further.
1111
1111
Op-code
Address 1
1111
1111
1111
Op-code
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COSC 3P92
Opcode Encoding
• Note that the three address fields may not
necessarily be used to encode a three-address
operand; they can be used as a single 12-bit oneaddress operand.
• Can have some part of the op-code to specify the
instruction format and/or length.
– if there are few two-address instructions, we may attempt to
make them shorter instead and to use the first two bits to
indicate the instruction length, e.g., 10 means two-address and
11 means three address.
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COSC 3P92
Op-code Encoding
• Huffman encoding
– Given the probability of occurrences of each instruction, it is
possible to encode all the instructions with minimal number of
bits, and with the following property:
Fewer bits are used for most frequently used instructions
and more for the least frequently used ones.
1
0
1/2
0
1
1
1/4
1
0
1/8
1/8
1/2
1/4
0
1
0
1
0
1
0
1
1/16
1/16
1/16
1/16
1/8
1/8
1/4
1/4
HALT
0000
JUMP
0001
SHIFT
0010
NOT
0011
AND
010
ADD
011
STO
10
LOAD
11
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COSC 3P92
Opcode encoding,
Huffman codes
• Huffman encoding algorithm:
– 1. Initialize the leaf nodes each with a probability of an instruction.
All nodes are unmarked.
– 2. Find the two unmarked nodes with the smallest values and mark
them. Add a new unmarked node with a value equal to the sum of
the chosen two.
– 3. Repeat step (2) until all nodes have been marked except the last
one, which has a value of 1.
– 4. The encoding for each instruction is found by tracing the path
from the unmarked node (the root) to that instruction.
• may mark branches arbitrarily with 0, 1
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COSC 3P92
Opcode encoding,
Huffman codes
• Advantage:
– minimal number of bits
• Disadvantage:
– must decode instructions bit-by-bit, (can be slow).
– to decode, must have a logical representation of the encoded
tree, and follow branches as you decipher bits
– Fact is, most decoding is done in parallel
– Gives a speed advantage
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COSC 3P92
Addressing modes
• inherent
– an op-code indicates the address of its operand
CLI
; clear the interrupt flag
• immediate
– an instruction contains or immediately precedes its operand
value
ADD #250, R1
% R1 := R1 + 250;
• Absolute/Direct
– an instruction contains the memory address of its operand
ADD 250, R1
% R1 := R1 + *(250);
• register
– an instruction contains the register address of its operand
ADD R2, R1
% R1 := R1 + R2;
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COSC 3P92
Addressing Modes
• register indirect
– the register address in an instruction specifies the address
of its operand
ADD @R2, @R1
% *R1 := *R1 + *R2;
• auto-decrement or auto-increment
– The contents of the register is automatically decremented
or incremented before or after the execution of the
instruction
MOV (R2)+, R1
MOV -(R2), R1
% R1 := *(R2); R2 := R2 + k;
% R2 := R2 - k; R1 := *(R2);
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COSC 3P92
Addressing Modes
• indexed
– an offset is added to a register to give the address of the
operand
MOV 2(R2), R1
% R1 := R2[2];
• base-register
– a displacement is added to an implicit or explicit base register
to give the address of the operand
• relative
– same as base-register mode except that the instruction pointer
is used as the base register
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COSC 3P92
Addressing modes
• Indirect addressing mode in general also applies
to absolute addresses, not just register
addresses; the absolute address is a pointer to
the operand.
• The offset added to an index register may be as
large as the entire address space. On the other
hand, the displacement added to a base register
is generally much smaller than the entire address
space.
• The automatic modification (i.e., auto-increment
or auto-decrement) to an index register is called
autoindexing.
• Relative addresses have the advantage that the
code is position-independent.
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COSC 3P92
Instruction Types
• Instructions, of most modern computers, may be
classified into the following six groups:
– Data transfer (40% of user program instructions)
MOV, LOAD
– Arithmetic
ADD, SUB, DIV, MUL
– Logical
AND, OR, NOT, SHIFT, ROTATE
– System-control
Test-And-Set
– I/O
Separate I/O space input/output
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COSC 3P92
Instruction Types
• Program-control
– may be classified into the following four groups:
– Unconditional branch
BRB
NEXT
% branch to the label NEXT
– Conditional branch
SOBGTR
R5, LOOP
% repeat until R5=0
ADBLEQ
R5, R6, LOOP
% repeat until R5>R6
– Subroutine call
CALL
SUB
RET
% push PC; branch to SUB
% pop PC
– Interrupt-handling
TRAP
% generate an internal interrupt
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COSC 3P92
Instruction types
• Typical branch instructions
– test the value of some flags called conditions.
– Certain instructions cause these flags to be set automatically.
• linkage registers
– Used in implementing a subroutine.
– Typically include the instruction pointer and stack pointer..
• The parameters passed between the caller and the called
subroutine are to be established by programming conventions.
– Very few computers support parameter-passing mechanisms in the hardware.
• An external interrupt may be regarded as a hardware generated
subroutine call
– Can happen asynchronously.
– When it occurs, the current state of the computation must be saved either by
» the hardware automatically
» or by a program (interrupt-service routine) control.
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COSC 3P92
Examples: Intel Pentium X
• back-compatible to 8088 (16 bit, 8 bit data bus),
8086 (16 bit), 80286 (16 bit, larger addr), 80386 (32
bit), ...
• Based on IA-32 (instruction archetecture 32bit)
• 3 operating modes:
1. real mode - acts like 8088 (unsafe -- can crash)
2. virtual 8086 - protected
3. protected - acts like Pentium II +
4 privilege levels too (kernel, user, ...)
• little endian words
• registers: [5.3]
– EAX, EBX, ECX, EDX - general purpose, but have special uses
(eg. EAX = arithmetic, ...)
– ESI, EDI, EBP, ESP - addr registers
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COSC 3P92
P2
Registers
ESegment
= Extended
Registers,
Pointers
to memory,
copying
registers
for
designed
for
backwards
and
manipulating
strings in
backwards
compatibility.
Points
to
base
of
current
compatibility
toaddress
older
Intel
memory
P4
uses
a
flat
stack
frame,
also
called
CPUs,
used for arithmatic.
space
the
frame
Stack pointerpointer
PC
PSW or Flags
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COSC 3P92
Ultra SPARC III
• Single linear 2^64 memory space
• Registers:
– 32 64-bit general regs, 32 FP regs
– global var regs: used by all procedures
– register windows: param. passing done via registers (more later on
RISC vs CISC)
– CSW – current window pointer (register set swapping for procedure
calls).
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COSC 3P92
Overview of the
UltraSPARC III
ISA Level (2)
Operation of the
UltraSPARC III
register windows.
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COSC 3P92
8051
• Runs in 1 mode,
–
–
–
–
Single process, directly interfacing h/w,
No OS.
Ram & Rom are (can be) on chip
4 sets of 8 GP registers,
» 2 bits in PSW determine which set is active.
» Interrupts do not cause a save of registers on a stack but a
context switch.
» Registers are directly mapped to memory
• R0 = 0x0000 in memory etc.
– 127 bit addressable memory locations
» Bits correspond nicely to switches and LED outputs
– Some specialized registers
» Interupts
» Timers
» All mapped to memory 128 to 255.
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COSC 3P92
Overview of the
8051 ISA Level
(a) On-chip memory organization for the 8051.
(b) Major 8051 registers.
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COSC 3P92
Pentium: Instruction formats
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COSC 3P92
Pentium: Instruction formats
• formats are complex, irregular, with variable-sized
fields (due to historical evolution)
• no memory-to-memory instructions
• 8088/286 - 1 byte opcode
• 386 - expanding 1111 -> 2 byte opcode
• Some fields:
– 2-bit MOD - 4 modes,8 regs, 8 combination regs
– 3 bit register REG, R/M
– SIB (scale, index, base) array manipulation codes
– 1,2,4 more bytes for operands, constants
• Not all registers, modes applicable to all
instructions: highly non-orthogonal
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COSC 3P92
Example: PDP-11
instruction formats
• CISC machine
• 16 bit instn size
• possibly 1 or 2 16 bit address words follow
• 8 modes, 8 regs -- regs 6 & 7 are stack, PC
• "orthogonal" addressing -- addressing and opcodes
are independent.
• some instns use expanding opcode
• x111 -> use longer opcode
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COSC 3P92
Example: UltraSPARC inst. formats
• 32-bit instructions; 31 RISC instructions
• first 2 bits help decode instruction format
• to encode a 32 bit constant, need to do it in 2
separate instructions!
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COSC 3P92
Example: Pentium
addressing
• 8088/286 are very non-orthogonal, and addressing
possibilities are arbitrary for different registers
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COSC 3P92
Example: Pentium Addressing
• 386 -- if 16-bit segments used, then use previous
- if 32-bit segments, use following...
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COSC 3P92
Addressing: Pentium
• new modes are more regular, general
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COSC 3P92
Addressing: Pentium
SIB
mechanism:
[5.27] --> arrays
•scale = 1, 2,
4, 8
•multiply
scale to
Index register
•adding to
Base register
•and then 8
or 32-bit
displacement
•
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COSC 3P92
Examples of addressing
PDP-11
5.33
• power comes from ability of addressing modes to
treat stack ptr, PC like any other registers
• eg. mode 6 with PC (reg 7)
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COSC 3P92
Examples of addressing
PDP-11
• power comes from
ability of addressing
modes to
treat stack ptr, PC like
any other registers
• eg. mode 6 with PC
(reg 7)
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COSC 3P92
Addressing
& PDP 11
• orthogonality permits many
variations with one opcode
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COSC 3P92
Example: UltraSPARC addressing
• all instructions use immediate or register addressing,
except those that address memory
• only 3 instructions address memory: Load, Store, and a
multiproc. synch
– use indirect addressing
• register: 5 bits tell which register
• 13 bit constants for immediate
Example: 8051 addressing
• 5 modes
–
–
–
–
–
Inherent (Accumulator)
Register
Direct
Register Indirect
Immediate
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COSC 3P92
Can be used due to
limited size of the CPU
Discussion of Addressing Modes
A comparison of addressing modes.
Not used, since
Direct addressing is
a slow memory
reference.
Cpu is not designed
for processing
arrays, main
purpose is i/o and
control
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COSC 3P92
Addressing: Discussion
•PDP-11 is clean, simple; some waste
•Pentium: specialized formats, addressing schemes
•386 - 32 bit addressing is more general
•RISC (Ultra): simpler instructions, fewer modes
•Compilers will generate required addressing, so a simple
scheme will suffice
•Specialized modes, formats makes instruction parallelism
(pipelining) more difficult
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COSC 3P92
Addressing: Discussion
• Compact Instructions
– Advantages.
» smaller resource usage
» faster fetch, execution
– Disadvantages
» reduce robustness
• Larger instructions:
– Advantages.
» simpler formats
• less constrained
– Disadvantages
» performance
» waste
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COSC 3P92
The end
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