1. INSTRUCTION SET ARCHITECTURE
CHAP 9

2. INSTRUCTION SET ARCHITECTURE
COMPLEX INSTRUCTION SET COMPUTERS (CISCs)
Provide hardware support for high-level language
Have compact programs
REDUCED INSTRUCTION SET COMPUTERS (RISCs)
Simple instructions
Flexibility
Faster execution

3. RISC Architecture
Memory accesses are restricted to load and store instructions, and data manuplation instructions are register to register
Addressing modes are limited in number
Instruction formats are all of the same length
Instructions perform elemantary operations

4. INSTRUCTION FORMATS FOR PIC16F877 (RISC)

5. PIC16F877 INSTRUCTION SET (RISC)

6. CISC Architecture
Memory access is directly available to most types of instruction
Addressing modes are substantial in number
Instruction formats are of different lenghts
Instructions perform both elemantary and complex operations

7. 8051 instruction set (CISC)

8. 8051 instruction set (CISC)

9. 8051 instruction set (CISC)

10. 8051 instruction set (CISC)

11. Instruction set architectures
CISC
Compact programs and conserve memory
Efficiencies in performance may result through a reductionin the number of instruction fetches from memory, compared to the number of elementary operations performed.
Because of the high memory accessibility, the register files in a CISC are smaller than in a RISC
Because of complexity of the instructions and the variability of the instruction formats, microprogrammed control is often used.
For speed, a pipelined control is used.
CISC instructions are converted to a sequence of RISC-like operations that are procesed by the RISC like pipeline.

12. Elementary computer instructions
There is a basic set of elementary operations that most computers include them
Basic set instructions categories:
Data transfer instructions
Data manupulation instructions
Program control instructions

13. Data Transfer Instructions
Moving data from one place in the computer to another without changing the content of the data.
Registers   Memory
Registers   Register
Memory   Memory
Assembly language abbreviation recommended by an IEEE standart. Different computers, however, may use different mnemonics for the same instruction name.

14. Stack Instructions
A stack is used in some electronic calculators and computers for evaluation of arithmetic expressions (low performance)
A stack in a computer typically handles only state information related to procedure calls and returns and interrupts.
PUSH: places new item onto the top of the stack.
POP: removes one item from the stack.
Stack pointer (SP) : points the TOS.

15. Stack Instructions PUSH R1 SP  SP – 1 M[SP]  R1 POP R1 R1  M[SP]
Two microinstruction needed for PUSH and POP

16. Independent versus Memory-Mapped I/O
Transfer data between processor registers and input-ouput devices.
Similar load, store. But the dest/source is external registers, such as Ports.
Independent I/O systems
The address ranges assigned to memory and I/O ports are independent from each other.
Computer has distinct input and output instructions (Ex. IN 378h, OUT 21h)
Memory-mapped I/O
Assigns a subrange of the memory addresses for addressing I/O.
So, no need distinct input or output instructions, because the same instructions are used for manupulating both memory and I/O data. (Ex. LD, ST)
Same instruction set serves for both memory and I/O access.

17. Data Manupulation instructions
Arithmetic instructions
Logical and bit manupulation instructions
Shift instructions.

18. Arithmetic instructions
Double precision arithmetic
“Add with carry” instructions

19. Logical and bit manupulation instructions
AND instruction also called a “mask”.
R1 =
Mask =
R1 AND Mask =
OR instruction also called a “bit set”.
R1 =
Mask =
R1 AND Mask =
XOR instruction also called a “bit complement”.
R1 =
Mask =
R1 AND Mask =

20. Shift instructions SHR instruction R1 = 00101100 0 00101100 Carry
SHR R1 = Carry=0
SHRA instruction (signed shift)
R1 =
Carry
SHR R1 = Carry=0
ROR instruction
R1 =
SHR R1 =
RORC instruction
R1 =
Carry
SHR R1 = Carry=0

21. Floating-point calculations
In many scientific calculations, the range of numbers is very large.
Floating-point notation has two parts; fraction (mantissa) and exponent.
Ex. Decimal number:
Fraction Exponent Scientific notation
 x 10+4
F x 10E
Only F and E are physically represented in computer, radix 10 and decimal point are not shown explicitly.
A floating point binary number uses radix 2, not radix 10.
Ex is represented as 8-bit fraction and 6 bit exponent;
Fraction Exponent
x = +( )2 x 2+4

22. Floating-point calculations
A floating point number is said to be normalized is the mostf significiant bit (Msb) of the fraction is non zero.
Ex (normalized) (not normalized)
A zero can not be normalized, all bits are 0’s.
Floating point represantation increases the range of numbers that can be accomodated in a given register.
48-bit register  ± (247-1)  ± 1014
48-bit register  35-bits fraction, 12-bits exponent  ± (1-2-35) x  10615
Bit numbers of fraction effects the presicion

23. Floating-point calculations
Arithmetic operations
x 102
x 10-1
Shift fraction until two exponents are equal. (shift left causes error, shift right decreases precision)
x 102
x 102
x 105
x 105
x 105  x 106 (normalized)
x 105
x 105  x 103 (normalized)
Multiply and divide do not require alignment of the fractions.

24. Floating-point calculations
Biased Exponent
The sign and fraction part of a floating point number is usually a signed-magnitude representetion. The exponent representation emplyoed in most computers is known as a biased exponent.
e = E + bias
Ex. Decimal exponent is –99 to Then bias is 99.
e = = 0 for –99
e = = 198 for +99
Floating point number contain only positive exonents.
It is then simpler to compare without concerning signs.

25. Floating-point calculations
Standart Operand Format
Arithmetic instructions that perform operations with floating-point data often use the suffix F (Ex. ADDF)
There are two standart formats for representing a floating-point numbers.
Single precision data types (32 bits) Mnemonic Suffix : FS
Double precision data types (64 bits) Mnemonic Suffix : FL

26. Floating-point calculations
Standart Operand Format
IEEE standart for single precision floating-point operand

27. Floating-point calculations
Standart Operand Format
IEEE standart for single precision floating-point operand
Significand
f field Significand Decimal equivalent
Even though the f field by itself may not be normalized, the significand is always normalized because it has a nonzero bit in the most significand position.
This 1 bit is not included explicitly in the format, bu must be inserted by the hardware during arithmetic computations.
(-1)s 2e-127 x (1.f )
x (2 – 2-23 ) Maximum number to represent

28. Floating-point calculations
(-1)s 2e-127 x (1.f )
e = 255 and f = 0 ; ± infinity
e = 255 and f  0 ; not a number ( invalid operations)
e = and f = 0 ; ± zero
e = and f  0 ; denormalized number

29. Program control instructions

30. Program control instructions
Conditional branch instructions

31. Program control instructions
Conditional branch instructions

32. Program control instructions
Procedure Call and Return Instructions
A procedure is also called subroutine
Call procedure instructions performs two operations:
Stores PC value in temporary register (like stack) as a return address.
Load PC with the procedure address
Call
SP  SP - 1
M[SP]  PC
PC  Efective address
Return
PC  M[SP]
SP  SP + 1

33. Program Interrupt
A program interrupt (kesme) is used to handle a variety of situations that require a departure from the normal program sequence.
A program interrupt transfers the control from a program that is currently running to another service program as a result of an externally- or internally generated request.
In principle interrupt procedure is similar to a call procedure
The interrupt usually initiated at an upredictable point in the program by an external or internal signal.
The address of the service program that processes the interrupt request is determined by a hardware procedure, rather than address field of an instruction
In response to interrupt, it is necessary to store information that defines all or part of the contents the register set, rather than storing only PC

34. Program Interrupt
After computer has been interrupted and the appropriate service program executed, the computer must return to exactly the same state that it was in before the interrupt occured.
Some computers store only the PC. Other storing operations must do by the user.
Most computers will not respond to an interrupt until the instruction which currently executed is completed. Then, just before going to fetch the next instruction, the control checks for any interut signals. If an interrupt has occured, control goes to a hardware interrupt cycle. During this cycle , the contents of some part or all of register set and PC are pushed onto stack.

35. Program interrupt Types of Interrupts: External interrupts
Input output devices
timing devices
from a circuit monitoring power supply
or any other external sources
Internal interrupts
Arises from illegal or erroneous use of an instruction or data
Called traps, overflow, divide by zero, stack overflow etc.
Software interrupts
Initiated by an instruction
A special call instruction that behaves like an interrupt rather than a procedure call. İt can be used by the programmer to initiate an interrupt procedure at any desired point in the program.
Example DOS interrupts functions (timer tick interrupt)

36. Processing external interrupts
Microinstructions to implement an interrupt
SP  SP –1
M[SP]  PC
SP  SP-1
M[SP]  PSR
EI  0
INTACK  1
PC  IVAD