Computer Org Architecture_Lecture 08.pptx
- 1. Computer Architecture Lecture 08 Instruction Set Instructor: Sultana Jahan Soheli Assistant Professor , ICE, NSTU
- 2. Reference Books • Computer Organization and Architecture: Designing for Performance- William Stallings (8th Edition) – Any later edition is fine
- 3. Machine Instruction Characteristics • Instructions it executes • The collection of different instructions that the processor can execute is referred to as the processor’s instruction set • 4 Basic elements of an machine instruction – Operation code – Source operand reference – Result operand reference – Next instruction reference
- 4. Elements of a Machine Instruction • 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 • 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
- 5. Elements of a Machine Instruction • The address of the next instruction could be either- – real address or – a virtual address • Depends on architecture • Source and result operands can be in one of four areas: • Main or virtual memory: As with next instruction references, the main or virtual memory address must be supplied • Processor register: With rare exceptions, a processor contains one or more registers that may be referenced by machine instructions – If only one register exists, reference to it may be implicit – If more than one register exists, then each register is assigned a unique name or number, and the instruction must contain the number of the desired register
- 6. Elements of a Machine Instruction • Immediate: The value of the operand is contained in a field in the instruction being executed • I/O device: The instruction must specify the I/O module and device for the operation – If memory-mapped I/O is used, this is just another main or virtual memory address
- 7. Instruction Representation • Each instruction is represented by a sequence of bits, within a computer • 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 • Some common examples- ADD Addition SUB Subtraction DIV Division LOAD Load data from memory STOR Store data to memory
- 8. Instruction Representation • Operands are also represented symbolically • Example: ADD R, Y
- 9. Instruction Types • A typical high level instruction – X = X + Y • How is it done in low level/machine language? • A computer should have a set of instructions that allows the user to formulate any data processing task • The set of machine instructions must be sufficient to express any of the instructions from a high-level language • Basic types- – Arithmetic instructions – Logic (Boolean) instructions – memory instructions – I/O instructions – Test instructions – Branch instructions
- 10. Number of Addresses • Arithmetic and logic instructions will require the most operands • Virtually all arithmetic and logic operations are either unary (one source operand) or binary (two source operands) • Thus, we would need a maximum of two addresses to reference source operands • The result of an operation must be stored, suggesting a third address, which defines a destination operand • Finally, after completion of an instruction, the next instruction must be fetched, and its address is needed • Zero-address instructions are applicable to a special memory organization, called a stack • Fewer addresses per instruction result in instructions that are more primitive, requiring a less complex processor
- 11. Number of Addresses • The design trade-offs involved in choosing the number of addresses per instruction are complicated by other factors • There is the issue of whether an address references a memory location or a register • Because there are fewer registers, fewer bits are needed for a register reference • Also, as we shall see in the next chapter, a machine may offer a variety of addressing modes, and the specification of mode takes one or more bits • 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
- 12. Instruction Set Design • Most fundamental issues include- • Operation repertoire: How many and which operations to provide, and how complex operations should be • Data types: The various types of data upon which operations are performed • Instruction format: Instruction length (in bits), number of addresses, size of various fields, and so on • Registers: Number of processor registers that can be referenced by instructions, and their use • Addressing: The mode or modes by which the address of an operand is specified
- 13. Types of Operands • Important general categories are- – Addresses – Numbers – Characters – Logical data
- 14. Types of Operations • General types- – Data transfer – Arithmetic – Logical – Conversion – I/O – System control – Transfer of control • Data Transfer: The data transfer instruction must specify several things – First, the location of the source and destination operands must be specified – Each location could be memory, a register, or the top of the stack – Second, the length of data to be transferred must be indicated – Third, as with all instructions with operands, the mode of addressing for each operand must be specified
- 15. Types of Operations • System Control – System control instructions are those that can be executed only while the processor is in a certain privileged state or is executing a program in a special privileged area of memory – Typically, these instructions are reserved for the use of the operating system • Transfer of Control • Why required? 1. In the practical use of computers, it is essential to be able to execute each instruction more than once and perhaps many thousands of times – It may require thousands or perhaps millions of instructions to implement an application – This would be unthinkable if each instruction had to be written out separately – If a table or a list of items is to be processed, a program loop is needed – One sequence of instructions is executed repeatedly to process all the data
- 16. Types of Operations 2. Virtually all programs involve some decision making – We would like the computer to do one thing if one condition holds, and another thing if another condition holds – For example, a sequence of instructions computes the square root of a number – At the start of the sequence, the sign of the number is tested – If the number is negative, the computation is not performed, but an error condition is reported 3. To compose correctly a large or even medium-size computer program is an exceedingly difficult task – It helps if there are mechanisms for breaking the task up into smaller pieces that can be worked on one at a time • Most common transfer-of-control operations found in instruction sets: branch, skip, and procedure call
- 17. Branch Instructions • A branch instruction, also called a jump instruction, has as one of its operands the address of the next instruction to be executed • Most often, the instruction is a conditional branch instruction • That is, the branch is made (update program counter to equal address specified in operand) only if a certain condition is met • Otherwise, the next instruction in sequence is executed (increment program counter as usual) • A branch instruction in unconditional branch • Two ways of doing this: which the branch is always taken is an – First, most machines provide a 1-bit or multiple-bit condition code that is set as the result of some operations – This code can be thought of as a short user-visible register – BRP X : Branch to location X if result is positive – BRN X :Branch to location X if result is negative
- 18. Branch Instructions • Another approach that can be used with a three-address instruction format is to perform a comparison and specify a branch in the same instruction • For example- • BRE R1, R2, X :Branch to X if contents of R1 = contents of R2 Fig: Branch Instructions
- 19. Skip Instructions • The skip instruction includes an implied address • Typically, the skip implies that one instruction be skipped; thus, the implied address equals the address of the next instruction plus one instruction length • Because the skip instruction does not require a destination address field, it is free to do other things • A typical example is the increment-and-skip- if- zero (ISZ) instruction
- 20. Procedure Call Instructions • A procedure is a self contained computer program that is incorporated into a larger program • At any point in the program the procedure may be invoked, or called • Used because of- – Economy and – Modularity • The procedure mechanism involves two basic instructions: • A call instruction that branches from the present location to the procedure, and a return instruction that returns from the procedure to the place from which it was called – Both of these are forms of branching instructions
- 21. Procedure Call Instructions • Important points: – A procedure can be called from more than one location – A procedure call can appear in a procedure • This allows the nesting of procedures to an arbitrary depth – Each procedure call is matched by a return in the called program
- 23. Addressing Techniques • Depends on trade-off between address range and/or addressing flexibility, and the number of memory references in the instruction and/or the complexity of address calculation 1) Immediate 2) Direct 3) Indirect 4) Register 5) Register indirect 6) Displacement 7) Stack
- 27. Addressing Techniques • Virtually all computer architectures provide more than one of these addressing modes – Often, different opcodes will use different addressing modes – Also, one or more bits in the instruction format can be used as a mode field • Effective address (EA): In a system without virtual memory, the effective address will be either a main memory address or a register • In a virtual memory system, the effective address is a virtual address or a register • The actual mapping to a physical address is a function of the memory management unit
- 28. Thank you!