Lecture 7: RISC computer

7 CISC and RISC
The design of computers either Cisc be or Risc depending on instruction set performed by the computer. The main differences between RISC and CISC are summarized by the following table:
Risc Cisc
Relatively few instructions large
Relatively few addressing mode Large No. of addressing mode
Fixed length of instruction Variable length of instruction
Decoding process is very simple Decoding process depends on instruction nature.
The operands must be in the registers The operands could be in the memory
Example: Apple hardware Example :Hardware of the Intel

Modern processors are pretty much all RISC, so, in this lecture we focus on RISC.
7-1 RISC ( Reduce Instruction Set Computer )


David Patterson
The new properties of Risc like ( fixed length of inst., limited addressing mode , the Presence (وجود) of operand in the register ) added new Features for Risc :-
1- Efficient pipe line utilization .
2- Efficient compiler.
3- Large No. of CPU registers. What is the advantage of this feature? What is the disadvantage?
4- Hardwired control unit rather than micro programmed.
5- Use the file of register to support the subroutine execution .

The access to memory variable is restricted (محدد او مقيّد) by two instructions (load and store) which are two address format . The rest of instruction are 3-address .

Example : Execute the following expression in Risc computer.
X = (A*Y) + (R*M)
Solution

Load R1,A
Load R2,Y
Load R3,R
Load R4,M

mul R1,R1,R2
mul R3,R3,R4
add R1,R1,R3
store X,R1







7-3 File of registers
Every subroutine has a Window, and each window has three registers (Local, Common, Global).The subroutine may use any one of these registers.
Local registers (L) are special registers used for defining the local variables in the subroutine , this is the reason why each subroutine has one L.
Common registers (C) are the registers shared among two or more subroutines.
Global registers (G) are general registers used for defining the global variables.







Example1: IF You have 5 subroutines and G = 10 , L = 6 , C = 4 . Answer the following :
1- Compute the length of register file.
2- Compute the size of each window.
SoL:
1- The length of register file= (6+4)*5+10 = 60
2- The size of each window = 6+(2*4)+10 =24
Example2: Designate a File of registers depending on the following information:
The length = 74 .
The No. of subroutines= 4 (A, B, C and D)
G=10 registers.
L= 10 registers.
C= 6 registers.
First separate G because it s for all Subroutines . Then, each subroutine has tow C and one L.