in1210/01-pds 1 tu-delft instructions and addressing

In1210/01-PDS 1TU-Delft

Instructions and addressing


Computer System

READ(X)READ(Y)ADD(X,Y,Z)WRITE(Z)

X: 1Y: 2Z: 3 • •

IR:

PC:

arithmeticunit

Central Processing Unit

controlunit

Main Memory

Input

Output

registers

data and instructions


Instruction and wordlengths(1)

instruction

instruction

instructioninstruction

012345678910

word

instructioninstructioninstruction instructioninstruction instruction

012345678910

wordaddress address




012345678910

wordaddress




012345678910

wordaddress

instructioninstr. instrinstruction instr. instrinstruction instr. instr

instructioninstr. instrinstruction instr. instrinstruction instr. instr


32 bit word formats

opcode specifier operand specifiers

31 30 1 0

byte byte byte byte

a two’s complement number

4 ASCII characters

32 bit

a machine instruction


Byte formats

0 1 2 34 5 6 78 9 - -- - - -- - - -

0

21

word

Big endian

e.g. PowerPC 68000

3 2 1 07 6 5 4- - 9 8- - - -- - - -

0

21

Little endian


Question What problems can occur in porting data

between machines with big-endian and little-endian storage?


Type of Instructions There are 4 types of instructions

- Data Copy operations» between memory and registers

» between memory locations

» between registers

- Arithmetic and Logic operations- Program flow control- I/O operations


Symbolic notation Copy instructions

[R1 ] M(LOC )

Arithmetic operations M(C) M(A) + M(B)

LOC, A, and B are memory addresses M(address) means contents of memory

location at address. [R] means contents of register R.


Operand specification formats Three address instructions

format: INSTR source#1,source#2,destination

example: Add A,B,C

means: M(C) M(B) + M(A) Two address instructions

format: INSTR source, destination

example: Add A,B

means: M(B) M(B) + M(A)


Two address instruction Two operand instructions destroy contents of

the B location Need other instruction to avoid that;

Move B,C We then have

Move B,C

Add A,C

meaning: M(C) M(B);

M(C) M(C) + M(A) ;


One address instructions One address have implicit source (often

called Accumulator)Load A

Add A

Store C

meaning

[Accu] M(A);

[Accu] [Accu] + M(A);

M(C) [Accu]


Registers Many computers have a number of General-

Purpose registers inside the CPU Access to registers is faster than to memory

locations Used to store temporary data during

processing Registers require less bits of address than

main memory


Register addressing Let Ri denote register General operation ADD A,B,C can be broken

down toMove A,R0

Add B,R0

Store R0,C

meaning [R0] M(A);

[R0] [R0] + M(B);

M(C) [R0]


Instruction formats (1)

opcode specifier operand specifiers

general format

opcode operand

one operand addressing


Instruction formats (2)

opcode operand operand

two operand addressing


Accumulator architecture

AccumulatorPC

CPU

MainMemory


Question

How many instructions can be defined when the opcode field is 5 bit ?


Example

instruction

accu

4 bits 12 bits

opcode operandm

15 0

sign bit


Instruction setCode Opcode Meaning

01000101011001111000100110101011

LoadStoreAddSubMulDivAndOr

[Accu] ← ( )M m( )M m ← [Accu]

[Accu] ← [Accu] + M( )m[Accu] ← [Accu] - M( )m[Accu] ← [Accu] * M( )m[Accu] ← [Accu] / M( )m[Accu] ← [Accu] & M( )m[Accu] ← [Accu] vM( )m

000000010010

J MPJSRRTS

Got omCa ll Subroutin e a t mRetur n from Subroutine

1100110111101111

JZJNZJ PJN

Got o mi [f Accu]= 0Got o mi [f Accu] =/ 0Got o mi [f Accu] > 0Got o mi [f Accu] < 0


Multiple register architecture

R0CPU

MainMemory

R1

R2

R3


Straight-line sequencing

…...…...

…….

Move A,R0Add B,R0Move R0,C • •

ii+4i+8

AB

C

address

Program for M(C) M(B) + M(A)


Straight-line sequencing

Add Nn,R0Move R0,S

Move N1,R0Add N2,R0Add N3,R0 • •

ii+4i+8

i+4n-4i+4n

address

Program for addition of n numbers


Branching

......

n

Clear R0Move N,R1

Decr R1Branch>0 LMove R0,S

L

SNN1

NnProgram for addition of n numbers

Determine address of“next” number and add itto R0

programloop


Common branch conditions N (negative) set to 1 of result is negative Z (zero) set to 1 of result is zero V (overflow) set to 1 of result overflows C (carry) set to 1 of carry-out results


Question

Why is the carry condition important?


Addressing modes Addressing modes determine how the address

of operands is determined Typical 4 addressing modes

- immediate addressing- direct addressing- indirect addressing- index addressing


Immediate addressing(1)

opcode specifier operand

instruction

ADD # -1JNZ 10 • •

10

example: simple counting loop


Immediate addressing(2) Advantages:

- no additional calculations needed to obtain operand- fast

Disadvantages:- Operand value must be known- Operand value cannot be changed- Limited no of bits available

Notation: MOVE #200,R0 Meaning: [R0] 200


Direct addressing(1)

opcode specifier mem or reg address

instruction

memoryor

registers

ADD 13JNZ 10 • # -1

10

13


Direct addressing(2) Advantages:

- Operand separate from instruction- Can be changed- Full word length available

Disadvantages:- More memory accesses- More storage occupation

Notation: ADD R1,R2 Meaning: [R2] [R2] +[R1] Also called “Absolute addressing” (Ham.)


Indirect addressing(1)

ADD (12)JNZ 10 13 # -1

10

13

opcode specifier mem or reg address

instruction

op. address

operand

memoryor

registers


Indirect addressing(2)

Advantages:- Actual address of operand is not in instruction- Can be changed

Disadvantages:- Even more memory or register references- More memory occupation

Notation: ADD (R1),R2 Meaning: [R2] [R2] + M([R1])


Example indirect addressing

programloop L

N

N1

Clear R0

Move N,R1

Move #N1,R2

Add (R2),R0

Add #4,R2

Decr R1

Branch>0 L

Move R0,S

n

S


Index addressing(1)

opcode Reg index

instruction

operand

memoryor

registers

operand

+

registers


Index addressing(2) Advantages:

- Allows specification of fixed offset to operand address

Disadvantages:- Extra addition to operand address

Notation: ADD X(R1),R3 (X=number) Meaning: [R3] [R3] + M([R1] + X)


Example index addressing

NE

Program with index addressing

programloop

sexagesalary

nEmpoyee IDsexagesalaryEmpoyee ID

L

Move #E,R0

Move N,R1

Clear R2

Add 8(R0),R2

Add #16,R0

Decrement R1

Branch>0 L

Div R1,R2

Move R2,Sum

Move N,R1


Additional modes Some computers have auto-increment

(decrement instructions) Example: (R0)+ Meaning ..M(R0)..; [R0] [R0]+1 Example: -(R0) Meaning [R0] [R0]-1; ..M(R0)..

in1210/01-pds 1 tu-delft instructions and addressing

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