Machine Instructions
Operations
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ITCS 3181 Logic and Computer Systems 2015 B. Wilkinson Slides4-1.ppt
Modification date: March 18, 2015
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).
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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.
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Some Key Features of Processor Used
1.Thirty-two 32-bit integer registers called R0 to R31
2. One register, R0, holds zero permanently
R31
R30
R29
R28
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
R4
R3
R2
R1
R0
0000 … 0000
R31 holds return
address for
procedures.
R29 is a stack
pointer see later.
store register
using register indirect addressing plus offset only:
LD/LB destination register, offset[source register]
ST/SB source register, offset[source register]
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Data Transfer
Instructions that copy the contents of one location to another
location.
Examples
Instruction
MOV R1,R2
LD R3,100[R2]
ST [R5],R4
Comments
;R1 = R2
;Contents of memory whose address is
;given by 100 + R2 copied to R3.
;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.
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Arithmetic Instructions
Performs an arithmetic operation such as addition, subtraction,
multiplication or division.
Examples
ADD R1,R2,R3
SUB R5,R4,3
;R1 = R2 + R3
;R5 = R4 – 3
For literals (immediate addressing) differences in assembly language
notation. Might be written as:
SUB R5,R4,#3
;R5 = R4 - 3
SBI R5,R4,3
;R5 = R4 - 3
or
depending upon assembly language.
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Logical Instructions
Performs bit-wise logical operation such as AND, OR, exclusiveOR, 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
z=z|2
;bit-wise AND y with the number 1
;bit-wise OR z with the number 2
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Machine Instruction Examples
AND R1,R2,R3
if
then
R2 = 10100010100101011000010100001111
R3 = 01011011111010100010100010111010
R1 = 00000010100000000000000000001010
OR R1,R2,R3
if
then
;R1 = R2 “AND” R3
;R1 = R2 “OR” R3
R2 = 10100010100101011000010100001111
R3 = 01011011111010100010100010111010
R1 = 11111011111111111010110110111111
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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
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Machine Instruction Examples
Examples
SHL R1,R1,1
X = 0 or 1 see next slide
;Shift R1 left one place
1 0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0 1
0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0 1 X
SHR R1,R1,1 ;Shift R1 right one place
1 0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0 1
X 1 0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0
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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.
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Example
Starting with a number 9
000 ... 0001001
Shift arithmetic left one place. Get 18
000 ... 0010010
Shift arithmetic right two places. Get 4
i.e. lost the 0.5.
000 ... 0000100
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Note: Java has logical shift right - called unsigned right
shift, >>>.
Example
x = x >>> 2;
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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).
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Question
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
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Rotate Instructions
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
1 0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0 1
0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0 1 1
ROR R1,R1,1 ;Rotate R1 right one place
1 0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0 1
Version of
rotate exists
where the
rotate
passes thro a
Carry flag
within the
condition
code
register, see
later about
the CCR
1 1 0 1 0 1 1 1 1 0 1 0 1 ... 1 1 0
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Control Flow
Compilers must translate statements such as:
if ((x != y) && (z < 0)) {
a = b + 5;
b = b + 1;
}
into machine instructions.
Unreasonable to try to provide a unique machine instruction for this
IF statement because of the vast number of possible IF statements.
Need to extract essential primitive operations for machine
instructions.
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Decompose into simple IF statements of the form:
if (x relation y) goto L1;
where:
relation is any of usual relations allowed in high level languages
(<, >, >=, <=, ==, !=)
L1 is a label prefixed to an instruction to identify it.
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i.e. translate
if ((x != y) && (z < 0)) {
a = b + 5;
b = b + 1;
}
...
into
Label
L1:
if (x == y) goto L1;
if (z >= 0) goto L1;
a = b + 5;
b = b + 1;
...
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Several ways of implementing the IF statement.
if (x relation y) goto L1;
Here we will consider two ways:
1. Using one branch instruction
Used in our design and lab.
2 Using two instructions, one to determine whether relationship
is true, and another to branch to L1 if true.
Used by Intel and based upon a very old instruction set, so we
have to know about it.
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1. Using one branch instruction
Single “branch” machine instruction compares two registers and
goes to the labeled location if the condition is true, i.e.
Bcond Rs1, Rs2, L1
changes the execution sequence to the instruction labeled L1 if
Rs1 cond Rs2 is true, where cond can be any of six possible
conditions, and Rs1 and Rs2 can be any of the 32 registers.
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Conditional Branch Instruction op-codes
Bcond
Bcond
BL
BG
BGE
BLE
BE
BNE
Condition
High level language notation
Branch if less than
<
Branch if greater than
>
Branch if greater or equal to
>=
Branch if less or equal to
<=
Branch if equal
==
Branch if not equal
!=
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Example
To implement
if (x == y) goto L1;
.
.
.
L1:
by a single machine instruction, we get:
BE R1,R2,L1
.
.
.
L1:
where the compiler allocates R1 for x and R2 for y.
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Machine Instruction Encoding
The instruction
Bcond Rs1,Rs2,L1
requires an op-code (Bcond), the two source registers Rs1 and Rs2,
and L1 to be specified.
Op-code
Bcond
Rs1
Rs2
L1
Note: Bcond is either BL, BG, BGE, BLE, or BNE
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Specifying “target” location L1
Several ways L1 could be specified in instruction:
(a) Absolute (direct) addressing
Address of L1 held in instruction.
(b) (PC) Relative addressing Distance from branch instruction to
labeled instruction stored in instruction.
Program counter, PC, holds the location of the instruction to be fetched from
memory next, see later.
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(a) Absolute (direct) addressing
BE R1,R2,120
.
.
.
L1:
;location 120 say
Op-code
Bcond
Rs1
Rs2
120
Note: Absolute addressing will not used in our design.
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(b) (Program counter) Relative Addressing
Specify target location as number of instructions from branch
instruction.
Reasoning:
• Mostly, conditional branch instructions used to implement small
changes in sequences or program loops of relatively short length,
so distance from branch to target (label) quite small compared to
full address of the labeled instruction.
• Also good programming practice to limit sequence changes to
short distance from current location to avoid difficult to read code.
• Also makes code relocatable. (i.e. code can be loaded anywhere in
memory without altering branch instructions.)
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PC-Relative Addressing
The number of locations to the target is held in the instruction as
an offset.
Branch op-code
Offset
Offset is added to the program counter to obtain the “effective
address” of the target location.
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(PC) Relative Addressing
Offset/displacement
BE R1,R2,+30 ; location 90 say
.
.
.
30
L1:
; location 120 (90 + 30)
Op-code
Bcond
Rs1
Rs2
We will deal with how to implement this later. Note: PC incremented by the size of
instruction, 4 here, after instruction fetched from memory in most implementations.
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Question
How would one code:
if (x > y) x = 10;
with machine instructions where x and y are stored in R2 and R3
respectively?
Answer
BG R2, R3, L1
MOV R2, 10
L1:
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2. Using two instructions, one to determine whether
relationship is true, and another to branch to L1 if true.
In this approach, we determine whether the Boolean condition in
if (x relation y) goto L1;
is true by subtracting y from x and recognizing whether the result is
positive or negative, zero, or not zero:
relation
<
>
>=
<=
==
!=
x-y
negative
positive and not zero
positive or zero
negative or zero
zero
not zero
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Condition Code Register (CCR)
Contains a set of flags (single bits) used to be able to recognize the
different possible relationships (<, >, >=, <=, ==, !=) after the
previous arithmetic operation.
Flags in condition code register indicate a particular aspect of the
result of the last arithmetic instruction, i.e. positive or negative,
zero or not zero, …
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Sign Flag (S or N flag)
(positive or negative)
Indicates whether previous arithmetic result is negative or positive.
S = 1 for negative result
S = 0 for positive result.
S is actually the most significant (sign) bit of the result of the
previous arithmetic operation
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Zero Flag
Zero flag, Z:
Z = 1 for result = 0
Z = 0 for result != 0
(Note zero is a positive number.)
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Condition Code Register
Condition Code register normally closely linked to the ALU
(Arithmetic and Logic Unit):
Processor
Internal buses
S – Sign flag
Z – Zero flag
Source operands
ALU
Answer
Condition code
register
S
Z
Sample allocation of bits
(There are other bits used not yet described)
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Using Condition Code Register
Decompose IF statement such as:
if (x relation y) goto L1
into two sequential operations:
1. Subtract y from x which sets condition code register according
to result
2. Read condition code register and “branch” to L1 if specific
condition indicated
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Step 1 Subtract and Set Condition Code Register
All arithmetic instructions set condition code register according to
the result of the arithmetic operation, but a compare instruction
specifically provided, similar to a subtract instruction except result is
not stored anywhere, only the CCR flags set according to the result.
Example
CMP R1, R2
;R1 - R2, sets CCR flags
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Step 2 Reading Condition Code Register and
Branching
A conditional branch instruction used to examine the values stored in
the condition code register to determine whether the specific
condition exists and to branch if it does.
All six conditions usually available:
BL
BG
BGE
BLE
BE (or BZ)
BNE (or BNZ)
Branch if less than
Branch if greater than
Branch if greater or equal to
Branch if less or equal to
Branch if equal
Branch if not equal
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Example
Suppose x held in R1 and y held in R2. The if statement:
if (x == y) goto L1;
could be implemented by sequence of two instructions, CMP and
BE:
CMP R1,R2
BE L1
;Compare contents of R1, R2 (x, y)
;Go to L1 if equal
L1:
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Example
Suppose x held in R1 and y held in R2. The if statement:
if (x == y) goto L1;
could be implemented by sequence of two instructions, CMP and BE:
Condition code register
Z = 1 if R1 - R2 = zero otherwise Z = 0
S
Z
Write
CMP R1,R2
BE L1
;Compare contents of R1, R2 (x, y)
;Go to L1 if equal
L1:
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Example
Suppose x held in R1 and y held in R2. The if statement:
if (x == y) goto L1;
could be implemented by sequence of two instructions, CMP and
BE:
Condition code register
Z = 1 if R1 - R2 = zero otherwise Z = 0
S
Z
Read
Write
CMP R1,R2 ;Compare contents of R1, R2 (x, y)
BE L1 ;Go to L1 if equal
L1:
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More complex constructions
if - then - else
Suppose we had to implement:
if (a < b) c = a; else a = c;
...
assuming that the variables a, b, and c are assigned to registers R1,
R2, and R3 respectively.
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Leads to:
CMP R1,R2
BL L1
MOV R1, R3
L1:
L2:
MOV R3,R1
...
;Compare R1, R2 (x, y)
;Go to L1 if a less than b
;a = c
Skip over c = a, unconditionally
;
c = a;
We need to alter the instruction sequence unconditionally. With the
instructions we know about so far, it could be done with:
CMP R1, R1
BE L2
;if (x == x) goto L2;
but there is a special instruction called a jump instruction to do it.
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JUMP Instructions
Causes an unconditional change of execution sequence to a new
location.
Necessary to implement more complicated IF constructs, FOR, and
WHILE loops.
Using J as the opcode mnemonic, the jump instruction is:
J L1
;goto to L1
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With jump instruction:
L1:
L2:
CMP R1,R2
BL L1
MOV R1, R3
J L2
Go to L2
MOV R3,R1
...
;Compare R1, R2 (x, y)
;Go to L1 if a less than b
;a = c
;c = a;
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For loops
Suppose we had to implement the C/Java code sequence:
for (i = 1; i < 10; i++)
a = a * i;
...
Let us assume i is held in register R1, and x is held in register R2.
Possible solution
L2:
L1:
MOV R1, 1
CMP R1,10
BGE L1
MUL R2,R2,R1
ADD R1,R1,1
J L2
...
;i = 1
;Compare R1 with 10
;Go to L1 if end of loop
;a = a * i
;increment i
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Jump Instruction with Register-Indirect
Addressing
An “address register” specified in instruction holds the address of
the location holding the next instruction to be executed.
Examples
J [R1]
;jump to location whose address in R1
J 100[R1]
;jump to location whose address given
;by R1 + 100
Target address specified as absolute address, rather than relative
address.
Used to implement high level language SWITCH statements. Also
return from procedures see next section.
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Avoiding Condition Code Register
It turns out that the CCR approach has disadvantages when one
tries to implement a high performance processor, and does not
really suit RISC designs.
Makes it much more difficult to design a high-performance
processor.
CCR approach requires two sequential instructions with no
instructions allowed in between that affect the CCR. (All
arithmetic/logic instruction affect CCR.)
For the most part we will use single instruction in our designs.
An alternative to the CCR is to use a general purpose register in
place of CCR to hold “conditions.”
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Simplified version of branch instruction
Bcond Rs1, L1
where Rs1 is compared against zero rather than Rs2.
Do subtract operation previous to this instruction, placing result
in Rs1.
49
Question
How would one code:
if (x > y) goto L1;
where x and y are stored in R2 and R3 respectively using previous
simplified version of branch instruction?
Answer
SUB R4, R2, R3
BG R4, L1
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One version of branch instruction in MIPS
processor (Lab)
Use a general purpose register set to 1 of 0 if one register is less
than another.
SLT Rs1,Rs2,Rs3
;Rs1 = 1 if Rs2 < Rs3
together with a simple branch instruction that only tests condition
equal or not equal.
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Questions
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