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Computer Architecture and organization ppt.
- 1. Computer Architecture and Organization InstructionSets: Addressing Modes and Formats
- 2. Addressing Modes • Immediate •Direct • Indirect • Register • Register Indirect • Displacement (Indexed) • Stack
- 3. Immediate Addressing • Operandis part of instruction • Operand = address field • e.g. ADD 5 —Add 5 to contents of accumulator —5 is operand • No memory reference to fetch data • Fast • Limited range
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- 5. Direct Addressing • Addressfield contains address of operand • Effective address (EA) = address field (A) • e.g. ADD A —Add contents of cell A to accumulator —Look in memory at address A for operand • Single memory reference to access data • No additional calculations to work out effective address • Limited address space
- 6. Direct Addressing Diagram AddressAOpcode Instruction Memory Operand
- 7. Indirect Addressing • Memorycell pointed to by address field contains the address of (pointer to) the operand • EA = (A) —Look in A, find address (A) and look there for operand • e.g. ADD (A) —Add contents of cell pointed to by contents of A to accumulator
- 8. Indirect Addressing • Largeaddress space • 2n where n = word length • May be nested, multilevel, cascaded —e.g. EA = (((A))) – Draw the diagram yourself • Multiple memory accesses to find operand • Hence slower
- 9. Indirect Addressing Diagram AddressAOpcode Instruction Memory Operand Pointer to operand
- 10. Register Addressing • Operandis held in register named in address filed • EA = R • Limited number of registers • Very small address field needed —Shorter instructions —Faster instruction fetch
- 11. Register Addressing • Nomemory access • Very fast execution • Very limited address space • Multiple registers helps performance —Requires good assembly programming or compiler writing —N.B. C programming – register int a; • c.f. Direct addressing
- 12. Register Addressing Diagram RegisterAddress ROpcode Instruction Registers Operand
- 13. Register Indirect Addressing •C.f. indirect addressing • EA = (R) • Operand is in memory cell pointed to by contents of register R • Large address space (2n ) • One fewer memory access than indirect addressing
- 14. Register Address ROpcode Instruction Memory OperandPointerto Operand Registers Register Indirect Addressing Diagram
- 15. Displacement Addressing • EA= A + (R) • Address field hold two values —A = base value —R = register that holds displacement —or vice versa
- 16. Displacement Addressing Diagram RegisterROpcode Instruction Memory OperandPointer to Operand Registers Address A +
- 17. Relative Addressing • Aversion of displacement addressing • R = Program counter, PC • EA = A + (PC) • i.e. get operand from A cells from current location pointed to by PC • c.f locality of reference & cache usage
- 18. Base-Register Addressing • Aholds displacement • R holds pointer to base address • R may be explicit or implicit • e.g. segment registers in 80x86
- 19. Indexed Addressing • A= base • R = displacement • EA = A + R • Good for accessing arrays —EA = A + R —R++
- 20. Combinations • Postindex • EA= (A) + (R) • Preindex • EA = (A+(R)) • (Draw the diagrams)
- 21. Stack Addressing • Operandis (implicitly) on top of stack • e.g. —ADD Pop top two items from stack and add
- 22. Pentium Addressing Modes •Virtual or effective address is offset into segment —Starting address plus offset gives linear address —This goes through page translation if paging enabled • 12 addressing modes available —Immediate —Register operand —Displacement —Base —Base with displacement —Scaled index with displacement —Base with index and displacement —Base scaled index with displacement —Relative
- 23. Pentium Addressing ModeCalculation
- 24. PowerPC Addressing Modes •Load/store architecture —Indirect – Instruction includes 16 bit displacement to be added to base register (may be GP register) – Can replace base register content with new address —Indirect indexed – Instruction references base register and index register (both may be GP) – EA is sum of contents • Branch address —Absolute —Relative —Indirect • Arithmetic —Operands in registers or part of instruction —Floating point is register only
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- 26. Instruction Formats • Layoutof bits in an instruction • Includes opcode • Includes (implicit or explicit) operand(s) • Usually more than one instruction format in an instruction set
- 27. Instruction Length • Affectedby and affects: —Memory size —Memory organization —Bus structure —CPU complexity —CPU speed • Trade off between powerful instruction repertoire and saving space
- 28. Allocation of Bits •Number of addressing modes • Number of operands • Register versus memory • Number of register sets • Address range • Address granularity
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