Instructions

Portability

• In addition to making hardware backward compatible, we have also made software portable.

• In describing software, “portable” means that the software can be run on a variety of computers. Another term used is independence, in this case machine or hardware independence. Assembly’s use of specifics of the registers makes it machine dependent.

• One has to program at a higher level to gain machine independence.

• Of course, someone must deal with the specifics of the hardware but it doesn’t have to be you (Layering).

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High-level language

• Languages which are above assembly language, known as high-level languages, hide the details of the hardware and thus are portable or hardware independent.

• Of course, high-level language are not directly executed. There is an in-between process or processes (compiling, translating, interpreting, linking, etc.) necessary before they can be executed.

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Generations• Another way of discussing this is to talk about

generations. • 1GL or first-generation language is machine

language.• 2GL or second-generation language is assembly. • 3GL or third-generation language is a "high-level"

programming language, Fortran, C, Java, Visual Basic, etc.

• 4GL or fourth-generation language is a more descriptive language, closer to human language usually used to query databases.

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5th Generation Language

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Instruction Set

• One aspect of processor design is to determine what instructions will be supported.

• There must be rules (syntax) for how instructions are expressed, so that the code can be parsed, one instruction distinguished from the next, the data (operand) separated from the action (operator).

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Number of operands• An example of syntax would be whether an

operator is binary or unary• Binary operators take two operands

– e.g. the Boolean operator AND– (x<y) AND (i=j)

• Unary operators take one operand– E.g. the Boolean operator NOT– NOT(x<y)

• Some ops are context dependent– - 5 (unary) versus 4-5 (binary)

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Operation versus Instruction

• An operation such as addition, which is binary (takes two operands), can be coded using instructions that are unary (one operand) if there is a default location implied such as Accumulator A.

• 5 + 6 becomes – Load 5 (places data in acc. A)– Add 6 (adds number in Acc. A to new number)

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Where does the answer go?• Whether it’s 5 + 6 or (Load 5, Add 6),

there’s the question of what to do with the result.

• We can again use the default location of Accumulator A to place the answer in, or we can include a third operand that indicates where the result should be placed.

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Store• Placing the result of an operation in memory is known

as storing it. Thus with unary instructions, we would have – Load 5– Add 6– Store 7

• The operand of the store is an address indicating where to store the answer, which is held in Accumulator A.

• Or one might have just one instruction with three operands– Add 7, 5, 6

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Addressing Modes

• The “7” in the previous example was clearly an address. But this raises the question as to what were the operands in the two previous instructions (Load and Add).

• For those instructions, the operand might have been the actual numbers one wanted added or the addresses of numbers one wanted added.

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More than one kind of add

• By “Add” one typically means the latter case on the previous slide. The operand is not the number to be added but the address of the number to be added. – (Think variables.)

• A designer can include a distinct add instruction, Add Immediate, in which the operand is the actual number to be added.

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Add Indirect

• In another version of addition, the operand is an address, and the data at that address is also an address, and the actual number to be added is located at the second address.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: XXX

The arrow indicates the program counter, we assume it has not executed the statement it points to.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: 5

The Load 4 instruction has been executed. The value at location 4 (which is a 5) has been loaded into the accumulator.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: 12

The Add Indirect 5 instruction has been executed. One goes to location 5 to find a value of 6. That 6 is an address, thus one goes to location 6 to find a value of 7 and that is added to the 5 waiting in the accumulator. The result of 12 is placed in the accumulator.

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A short program

Address Value

0 LOAD 4

1 ADD INDIRECT 5

2 ADD IMMEDIATE 6

3 STOP

4 5

5 6

6 7

7 8

Acc. A: 18

The Add Immediate 6 instruction has been executed. The value 6 is data which is added to the 12 waiting in the accumulator. The result of 18 is placed in the accumulator.

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The Stop Instruction• Recall that in what is actually executed (machine

code, not assembly) the instructions themselves are numbers.

• Thus it is crucial to know within a single instruction which numbers correspond to an operation and which numbers are operands.

• Similarly on the level of the program itself, the processor needs to know where the program ends as there may be data stored after it.

• In a machine with an operating system, it is more a notion of returning (control) than of stopping or halting.

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Recap so far

• So there were issues about the number of operands. – Recall that we have a fetch-execute cycle – first an

instruction is retrieved from memory and then acted upon.

– With unary instructions adding two numbers and storing the result required three instructions, that’s three fetches and three executions.

– With ternary instructions it can be done with one instruction, one fetch and one execute. The execution is now more complicated but we have saved time on fetches.

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Recap so far (Cont.)

• More operators means more complicated circuitry, the load and store aspects of the instruction would have to built into each separate instruction.

• There is a speed versus complexity issue. And complexity also brings the issue of cost along with it.

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Recap so far (Cont.)• After determining the number of operands, came

the issue of what the operands mean. – Are they data, addresses of data, or addresses of

addresses of data?

• Either we can decide to support all of these types of instructions (addressing modes) and choose complexity. Or we can choose to support only some of them and sacrifice efficiency. – You can eliminate Add Immediate if you always store

the values you want to add.

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Data Types

• Apart from addressing, another issue is the type of data the operation is acting on. – The process for adding integers is different from the

process for adding floating point numbers. – So one may have separate ADD and FADD for the addition

of integers and floats respectively. – Furthermore, one may need to add instructions to convert

from one type to another. • To add an integer to a float, convert the integer to a float and then

add the floats.

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References

• Computer Architecture, Nicholas Carter• Digital Computer Electronics, Albert P.

Malvino and Jerald A. Brown• http://www.webopedia.com