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INSTRUCTION SET ARCHITECTURES

INSTRUCTION SET ARCHITECTURES. Chapter 5. Introduction. High-level languages hide it So, why do we need? Efficient programming Understanding the machine architecture. Instruction Formats. Architectures can be differenced by the following: Number of bits (16, 32, 64)

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INSTRUCTION SET ARCHITECTURES

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  1. INSTRUCTION SET ARCHITECTURES Chapter 5

  2. Introduction • High-level languages hide it • So, why do we need? • Efficient programming • Understanding the machine architecture

  3. Instruction Formats • Architectures can be differenced by the following: • Number of bits (16, 32, 64) • Operand storage in CPU (data in stack or registers) • Number of explicit operands per instruction (0-3) • Operand location (register-to-register, register-to-memory, memory-to-memory) • Operations (types and access restrictions) • Type and size of operands

  4. Designing decisions for instruction sets • Define the instruction set format is very important • Factors to consider: • Amount of space required by program • Complexity of the instruction set • Length of instructions • Number of instructions

  5. Continuing… • How to choose the design? • Short instructions are typically better (but reduces possibilities) • Instructions of fixed length (easy to decode but waste space) • Byte addressable • Fixed length does not imply fixed number of operands • Addressing modes • Little or big endian? • Registers (how many, and how to use)

  6. Little versus big endian • The way that computer stores bytes from multiple-byte data element • Little endian = store lower significant byte at lower address) • Big endian = guess… • UNIX usually big, PCs usually little • Intel always little, Motorola always big

  7. OK, but what does it mean? • If you have an integer:

  8. Pros | Cons

  9. Internal storage in CPU • Stack • Use stack to execute instructions • Operands found on top • Don’t allow random access (Of course, it is a stack) • Accumulator • Minimize complexity • Short Instructions • General Purpose Register (GPR) • Most widely accepted • Fast • Easy to deal

  10. Continuing… • GPR architecture can be broken in three • Memory-memory • Digital Equipment’s VAX • Register-memory • Intel, Motorola • Load-store • Data must be stored in register before operations are performed • SPARC, MIPS, Alpha, PowerPC

  11. Number of operands and instruction length • Fixed length • Wastes space but is fast • Variable length • More complex, but saves storage space • Instruction need to be aligned with words • Common formats • OPCODE only • OPCODE + 1 address (usually a memory) • OPCODE + 2 addresses (usually registers, or one register, one memory) • OPCODE + 3 addresses (usually registers, or combination of registers and memory)

  12. Continuing… • Machine instructions that have no operands must have a stack (where operations are made from top) to perform a operation that require one or two operands 2 3 5 2 3 3 5

  13. Continuing… • The same algorithm before would be like this according to the number of addresses: • Note that in the examples I presume that Z is initially 0

  14. Expanding opcodes • If you have a machine with “short opcodes”, you can use part of the “memory addressing bits” to expand opcodes (some opcodes do not need addresses), here is an example: • You have a machine with 16-bit instructions and 16 registers, you can use 4 bits to opcodes with 12 bits to addresses or 8 bits to opcodes and 8 bits to addresses or 12 bits to opcodes and 4 bits to address or only opcodes

  15. Continuing… • 15 3-addresses codes (4 bits to opcodes, 12 bits to addresses) • 0000 R1 R2 R3 • 1110 R1 R2 R3 • 14 2-addresses codes (8 bits to opcodes, 8 bits to addresses) • 1111 0000 R1 R2 • 1111 1101 R1 R2 • 31 1-addresses codes (12 bits to opcodes, 4 bits to addresses) • 1111 1110 0000 R1 • 1111 1111 1110 R2 • 16 0-addresses codes (16 bits to opcodes, 0 bits to addresses) • 1111 1111 1111 0000 • 1111 1111 1111 1111

  16. Instruction types • Data movement • Arithmetic • Boolean • Bit manipulation • I/O • Transfer of control • Special purpose

  17. Data Movement • Most frequently used instructions. Data is moved from memory into registers, registers to registers, registers to memory. • May be instructions to move between memory and register, one only for registers • RISC limits the instructions that can move to memory • Many machines have different instructions to bytes and words • MOVE, LOAD, PUSH, POP, EXCHANGE and variations

  18. Arithmetic operations • Instruction that deal with numbers (integer and float) • Many instructions sets provides different instructions for different data sizes (improve performance) • Many times operands are implied • Use the flag register (to report errors like overflow, underflow, use as carry) • ADD, SUBTRACT, MULTIPLY, DIVIDE, INCREMENT, DECREMENT, NEGATE

  19. Boolean logic instructions • Perform Boolean operations • Allow bits to set, cleared and complemented • Commonly used to control I/O devices • Affect flag register • AND, NOT, OR, XOR, TEST, COMPARE

  20. Bit manipulation instructions • Used for setting and resetting a bit or a group of bits within a given data word • Include arithmetic and logical SHIFT and ROTATE instructions (to left and right)

  21. Input/Output instructions • Input – transports data from a device or port to a register or memory • Output – transports data from register or memory to a device or port • May be different for characters and numbers

  22. Instructions for transfer of control • Alter the normal sequence of program execution • Branches, skips, procedure calls, returns, program termination • Jump, conditional jump

  23. Special purpose instructions • String processing, high-level language support, protection, flag control, word/byte conversions, cache management, register access, address calculation, no-ops, any other instructions that don’t fit in the previously mentioned

  24. Instruction set orthogonality • Don’t create instructions that duplicate other instructions, that would be a waste • Makes writing a language compiler easier, but have quite long words, with means the programs will be bigger and use more memory

  25. Addressing • Data types • Addressing modes

  26. Data types • Numeric data (integers and floating point numbers) • Signed / Unsigned • Various lengths • Might be different instructions for different lengths • Nonnumeric (strings, Booleans, pointers) • String • Copy, move, search, modify • Boolean • AND, OR, XOR, NOT • Pointers are addresses in memory

  27. Address modes • Allow specify where instructions operands are located • Immediate addressing • Direct addressing • Register addressing • Indirect addressing • Indexed addressing • Stack addressing (the operand is assumed to be on stack)

  28. Immediate addressing • Data to be operated is part of instruction • Very fast because it is included with instruction • Not very flexible because it is loaded at compile time

  29. Direct addressing • The value to be referenced is obtained by specifying its memory address directly to instruction • Fast, not part of instruction, but the value in the address is loaded to the register • Much more flexible

  30. Register addressing • A register, instead of memory, is used to specify the operand. • Very similar to direct addressing but the address is from a register, so it is faster

  31. Indirect addressing • Very powerful • Very flexible • Bits in address field specify a memory address of a pointer, the actual address of the operand is found by going to the address referenced by the pointer • There is also register indirect addressing, which is quite similar, but uses a register to point the data

  32. Indexed addressing • There is an index register and the address of the data we want is found by the sum of the value in the register to the address in the instruction • There is also based addressing, which is very similar, but the opposite, the register holds the base address and the value given is the index • Very useful to access arrays, and strings

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