1. Machine Instructions Operations
ITCS 3181 Logic and Computer Systems B. Wilkinson Slides4-1A.ppt Modification date: March 18, 2015

2. Instructions
We will use a simple instruction formats of a so-called reduced instruction set computer (RISC), which has the characteristics:
Simple, fixed length instruction format
A few addressing modes
Limited number of operations
Designed to achieve high speed of execution
It was recognized in the 1980’s that such processors would actually execute programs faster that the prevalent CISCs (complex instruction set computers).

3. Instruction format
Our instruction format is quite similar to the G4/G5 PowerPC and SUN Sparc processors and also similar but not identical to that used in the assembly language simulator in the labs.
Intel 64/IA32 processor family -- uses a very complex and archaic instruction format, based upon the early 8086 processor (which was themselves loosely based upon even earlier Intel processors).
However Intel processors now convert this complex instruction format (CISC) internally to simpler RISC formats for performance reasons.

4. Some Key Features of Processor Used
Thirty-two 32-bit integer registers called R0 to R31
2. One register, R0, holds zero permanently
3 All arithmetic done only between registers:
- Three-register format
op-code, destination register, source register 1, source register 2 or
- Immediate addressing
op-code destination register, source register, constant
4. Memory operations limited to:
load register, and
store register
using register indirect addressing plus offset only:
LD/LB destination register, offset[source register]
ST/SB source register, offset[source register] R0 R1 R2
R31
R30 R3
R29 R4
R28
0000 … 0000
R31 holds return address for procedures.
R29 is a stack pointer see later.

5. Data Transfer Examples Instruction Comments
Instructions that copy the contents of one location to another location.
Examples
MOV R1,R2 ;R1 = R2
LD R3,100[R2] ;Contents of memory whose address is
;given by R2 copied to R3.
ST [R5],R4 ;Contents of R4 copied to memory loc.
;whose address held in R5. (Offset = 0)
Note LD and ST cause 32-bit transfers.
Use LB and SB to cause 8-bit transfers.
Instruction
Comments

6. Arithmetic Instructions
Performs an arithmetic operation such as addition, subtraction, multiplication or division.
Examples
ADD R1,R2,R3 ;R1 = R2 + R3
SUB R5,R4,3 ;R5 = R4 – 3
For literals (immediate addressing) differences in assembly language notation. Might be written as:
SUB R5,R4,#3 ;R5 = R4 - 3 or
SBI R5,R4,3 ;R5 = R4 - 3
depending upon assembly language.

7. C/Java Language Examples
Logical Instructions
Performs bit-wise logical operation such as AND, OR, exclusive-OR, or NOT. AND, OR, exclusive-OR operate upon pairs of bits of two operands.
Bit-wise AND, OR, Ex-OR, and NOT are available in C and Java (although you probably did not come across them!):
C/Java Language Examples
y = y & 1 ;bit-wise AND y with the number 1
z = z | 2 ;bit-wise OR z with the number 2

8. Machine Instruction Examples
AND R1,R2,R3 ;R1 = R2 “AND” R3
if R2 =
R3 =
then R1 =
OR R1,R2,R3 ;R1 = R2 “OR” R3
then R1 =

9. C/Java language Examples
Shift Instructions
Moves the bits of a location one or more places left or right.
Again available in C/Java (although you probably did not come across them!):
C/Java language Examples
y = y << 1 ;shift y 1 place left 1
z = z >> 2 ;shift z 2 places right

10. Machine Instruction Examples
SHL R1,R1,1 ;Shift R1 left one place
SHR R1,R1,1 ;Shift R1 right one place
X = 0 or 1 see next slide

11. Arithmetical and Logical Shifts
Two types of shift usually provided:
“Logical” shift (SHL, SHR)
Fill free end with 0, i.e. X = 0.
“Arithmetic” shift (SAL, SAR)
Arithmetic shifts multiple/divide by 2.
Arithmetic shift right maintains value of sign bit, i.e. X = value of original sign bit.

12. Example Starting with a number 9 000 ... 0001001
Shift arithmetic left one place. Get
Shift arithmetic right two places. Get
i.e. lost the 0.5.

13. Note: Java has logical shift right - called unsigned right shift, >>>.
Example
x = x >>> 2;

14. Question
What is the difference, if any, between arithmetic shift left and logical shift left, i.e. what is the difference, if any, between:
SHL R1, R1, 1
and
SAL R1, R1, 1
Answer
Arithmetic shift left same as logical shift left (except arithmetic overflow may be detected).

15. Question Answer What is the effect of the sequence? SAR R1, R1, 1
SAL R1, R1, 1
Answer
Makes number even if odd, i.e. 5 would become 4

16. Rotate Instructions Examples
Moves bits of location one or more places left of right in a circular fashion.
Examples
ROL R1,R1,1 ;Rotate R1 left one place
ROR R1,R1,1 ;Rotate R1 right one place
Version of rotate exists where the rotate passes thro a Carry flag within the condition code register, see later about the CCR

17. Questions