COMPUTER ORGANIZATION & ARCHITECTURE Instructions Sets Architecture Lesson 5a 1
What are Instruction Sets The complete collection of instructions that are understood by a CPU Can be considered as a functional spec for a CPU Implementing the CPU in large part is implementing the machine instruction set Machine Code is rarely used by humans Binary numbers / bits Machine code is usually represented by human readable assembly codes In general, one assembler instruction equals one machine instruction 2
The Instruction Set Architecture (ISA) view of a machine corresponds to the machine and assembly language levels. A compiler translates a high level language, which is architecture independent, into assembly language, which is architecture dependent. An assembler translates assembly language programs into executable binary codes. 3
Elements of an Instruction Each instruction must contain the information required by the processor for execution These elements are as follows: Operation code: Specifies the operation to be performed (e.g., ADD, I/O). The operation is specified by a binary code, known as the operation code, or opcode. Source operand reference: The operation may involve one or more source operands, that is, operands that are inputs for the operation. 4
Result operand reference: The operation may produce a result. Next instruction reference: This tells the processor where to fetch the next instruction after the execution of this instruction is complete Source and result operands can be in one of four areas: Main or virtual Memory Processor register Immediate I/O Device 5
THE FETCH-EXECUTE CYCLE The steps that the control unit carries out in executing a program are: (1) Fetch the next instruction to be executed from memory. (2) Decode the opcode. (3) Read operand(s) from main memory, if any. (4) Execute the instruction and store results, if any. (5) Go to step 1. This is known as the fetch-execute cycle. 6
Instruction Representation Within the computer, each instruction is represented by a sequence of bits. The instruction is divided into fields, corresponding to the constituent elements of the instruction. Opcodes are represented by abbreviations, called mnemonics, that indicate the operation. Common examples include ADD, SUB, MUL, DIV, LOAD, STOR 7
Instruction Types A machine language expresses operations in a basic form involving the movement of data to or from registers. The set of machine instructions must be sufficient to express any of the instructions from a high-level language. We can categorize instruction types as follows: Data processing: Arithmetic and logic instructions Data storage: Movement of data into or out of register and or memory locations Data movement: I/O instructions Control: Test and branch instructions 8
Number of Addresses Processor architecture can be describe in terms of the number of addresses contained in each instruction What is the maximum number of addresses one might need in an instruction? 9
3 Addresses Two source operand locations and a destination operand Not common Needs very long words to hold everything Assume we which to compute Y = (A - B)/[C + (D X E)] 10
2 Addresses One address doubles as operand and result. Reduces length of instruction. Requires some extra work. -Temporary storage to hold some results Compute Y = (A - B)/[C + (D X E)] 11
1 Address Implicit second address Usually a register (accumulator) Common on early machines Computing Y = (A - B) / [C + (D X E)], we have; 12
Try This 1. X = (A + B * C)(D - E * F) 2. A = B * C + D Using the various types of number addressing, represent the following as; -1 address -2 addresses -3 addresses 13
HOW MANY ADDRESSES? More addresses More complex (powerful?) instructions More registers Inter-register operations are quicker Fewer instructions per program Fewer addresses More instructions per program Less complex instructions hence requires a less complex processor Faster fetch/execution of instructions 14
There is some threshold between one-address and multiple address instructions. With one-address instructions, the programmer generally has available only one general-purpose register, the accumulator. With multiple-address instructions, it is common to have multiple general purpose registers. Allows some operations to be performed solely on registers because register references are faster than memory references. 15
Instruction Set Design Most interesting, and most analyzed, aspects of computer design Very complex because it affects so many aspects of the computer system Defines many of the functions performed by the processor 16
Operation repertoire Design Decision/Issues How many ops? What can they do? How complex are they? Data types Various types of data upon which operations are performed Instruction formats Length of op code field Number of addresses 17
Registers Design Decision/Issues cont Number of CPU registers available Which operations can be performed on which registers? Addressing modes RISC v CISC 18
Types of Operands Machine instructions uses data for its operations. The most general categories are; Addresses Numbers Integer/floating point Characters ASCII etc. Logical Data Bits or flags 19
INTEL x86 DATA TYPES The x86 can deal with data types of 8 bit Byte 16 bit word 32 bit (double word) 64 bit (quad-word) 128 bit (double quad-word) Addressing is by 8 bit unit The x86 supports a lot of specific data types. 20
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TYPES OF OPERATIONS Number of different opcodes varies widely from machine to machine. Same general types of operations are found on all machines. Operations can be categorized as: -Data Transfer -Arithmetic -Logical -Conversion -I/O -System Control -Transfer of Control 22
DATA TRANSFER Data transfer instruction must specify several things -Source and Destination -Length of data -Amount of data May be different instructions for different movements e.g. IBM 370 Or one instruction and different addresses e.g. VAX 23
ARITHMETIC Most machines provide the basic arithmetic operations of add, subtract, multiply, and divide. These are invariably provided for signed integer (fixed-point) numbers. Often they are also provided for floating-point and packed decimal numbers. Other possible operations include a variety of single-operand instructions; for example, Absolute: Take the absolute value of the operand. Negate: Negate the operand. Increment: Add 1 to the operand. Decrement: Subtract 1 from the operand 24
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LOGICAL Machines also provide a variety of operations for manipulating individual bits of a word This form of operation can be term as bit twiddling. They are based on Boolean operations like AND, OR, NOT, XOR, which are also used as operation names. Some other op names for logical operations are Test, Compare, Shift, Rotate, Set control variables. 26
CONVERSION These instructions are the one that change the format or operate on the format of the data. Examples are Translate, Convert An example of a more complex editing instruction is the EAS/390 Translate (TR) instruction This instruction can be used to convert from one 8- bit code to another, and it takes three operands: TR R1 (L), R2 27
ADDRESSING MODES A program operates on data that reside in the computer s memory. These data can be organized in a variety of ways Programmers use organizations called data structures to represent the data used in computations. These include lists, linked lists, arrays, queues, etc The different ways in which the location of an operand is specified in an instruction are referred to as addressing modes 28
ADDRESSING MODES Cont There are various categories of addressing modes. These include; Immediate Direct Indirect Register Register indirect Displacement Stack The following notations are used in the next slides; A = contents of an address field in the instruction (X) = contents of memory location X or register X EA = actual (effective) address of the location containing the referenced operand R = contents of an address field in the instruction that refers to a register 29
IMMEDIATE ADDRESSING MODE This is the simplest form of addressing where the operand value is present in the instruction Operand = A It is used to define and use constants or set initial values of variables For example, the instruction Move 200 immediate, R0 places the value 200 in register R0 In assembly language this can be represented as Move # 200,R0 30
IMMEDIATE ADDRESSING MODE cont Advantage No memory reference other than the instruction fetch is required to obtain from the operand, thus saving one memory or cache cycle in the instruction cycle Disadvantage The size of the number is restricted to the size of the address field, which, in most instruction sets, is small compared with the word length. 31
DIRECT ADDRESSING MODE A very simple form of addressing is direct addressing, in which the address field contains the effective address (EA) of the operand. EA = A It s a common technique used in earlier generations of computers but is not common on contemporary architectures. It requires only one memory reference and no special calculation. Its limitation is that it provides only a limited address space 32
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INDIRECT ADDRESSING MODE In indirect addressing, the address field refer to the address of a word in memory, which in turn contains a full-length address of the operand. EA = (A) As defined earlier, the parentheses are to be interpreted as meaning contents of. 34
INDIRECT ADDRESSING MODE Advantage For a word length of N, an address space of 2N is now available. Disadvantage Instruction execution requires two memory references to fetch the operand, thus, one to get its address and a second to get its value Although the number of words that can be addressed is now equal to 2N the number of different effective addresses that may be referenced at any one time is limited to 2K where K is the length of the address field. 35
REGISTER ADDRESSING MODE Register addressing is similar to direct addressing. The only difference is that the address field refers to a register rather than a main memory address: EA = R Advantages Only a small address field is needed in the instruction No time-consuming memory references are required Disadvantage Address space is very limited. 36
REGISTER INDIRECT ADDRESSING MODE Register indirect addressing is similar to indirect addressing. The only difference is whether the address field refers to a memory location or a register. Thus, for register indirect address; EA = (R) Advantages and limitations of register indirect addressing is the same as for indirect addressing In addition, register indirect addressing uses one less memory reference than indirect addressing 37
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DISPLACEMENT ADDRESSING MODE A very powerful mode of addressing combines the capabilities of direct addressing and register indirect addressing. It is known by a variety of names depending on the context of its use This is referred to as displacement addressing: EA = A + (R) 39
DISPLACEMENT ADDRESSING MODE Cont Displacement addressing requires that the instruction have two address fields, at least one of which is explicit. The value contained in one address field (value = A) is used directly. The other address field, or an implicit reference based on opcode, refers to a register whose contents are added to A to produce the effective address 40
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