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Instruction Sets: Addressing modes and Formats

Instruction Sets: Addressing modes and Formats. Group #4 Eloy Reyes Rafael Arevalo Julio Hernandez Humood Aljassar Computer Design EEL 4709c Prof: Herman Watson Spring 2009. Addressing Modes. Immediate Direct Indirect Register Register Indirect Displacement (Indexed) Stack.

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Instruction Sets: Addressing modes and Formats

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  1. Instruction Sets:Addressing modes and Formats Group #4 • Eloy Reyes • Rafael Arevalo • Julio Hernandez • HumoodAljassar Computer Design EEL 4709c Prof: Herman Watson Spring 2009

  2. Addressing Modes • Immediate • Direct • Indirect • Register • Register Indirect • Displacement (Indexed) • Stack

  3. Immediate Addressing • Operand is part of instruction • Value to be loaded is fixed at compile-time (not very flexible) • e.g. ADD 8 • Add 8 to contents of accumulator • 8 is operand • Does not have an effective address • Fast (Value to be loaded – included in inst) • Value stored in memory follows immediately the operation code in memory

  4. Immediate Addressing Diagram Instruction Opcode Operand

  5. Direct Addressing • Address field contains address of operand • Effective address (EA) = address field (A) • e.g. ADD C • Add contents of cell C to accumulator • Look in memory at address C for operand • Directly retrieves stored value in memory from another memory location. • Fast (Internal RAM) • Limited address space

  6. Direct Addressing Diagram Instruction Opcode Address A Memory Operand

  7. Indirect Addressing • Memory cell pointed to by address field contains the address of (pointer to) the operand • EA = (C) • Look in C, find address (C) and look there for operand • Exceptional level of flexibility. • Always refers to Internal RAM.

  8. Indirect Addressing • 2n where n = word length • Multilevel, cascaded, or nested • Address space (Large) • Slower • Multiple memory accesses -> operand

  9. Indirect Addressing Diagram Instruction Opcode Address A Memory Pointer to operand Operand

  10. Register Addressing • Operand contained in register (Named -> address filed) • Registers (Limited) • Needs -> Very small address field • Instructions (Fast, hence faster instruction fetch) • EA= R

  11. Register Addressing • Address space -> Very limited • Memory Address -> None • Multiple registers -> Improves performance • N.B. C programming • Good compiler writing or assembly programming -> Required • Execution -> Very fast

  12. Register Addressing Diagram Instruction Opcode Register Address R Registers Operand

  13. Register Indirect Addressing • Address of operand obtained indirectly via an address register. • Indirect addressing uses an address register to point at the location of the operand in memory. • EA = (R) • Generates lots of different addresses when program executed. • Address Space -> Large (2n)

  14. Register Indirect Addressing Diagram Instruction Opcode Register Address R Memory Registers Operand Pointer to Operand

  15. Displacement Addressing • Choosing displacement field size is important -> directly affects instruction length. • EA = A + (R) • Address field hold two values • A = base value • R = register that holds displacement • or vice versa

  16. Displacement Addressing Diagram Instruction Address A Opcode Register R Memory Registers Pointer to Operand Operand +

  17. Relative Addressing • An address specified by indicating its distance from another address. (Base Address) • A version of displacement addressing • For example: B+15 • B being the base address and 15 the distance (Called the offset)

  18. Base-Register Addressing • R -> Explicit or Implicit • R contains pointer to base address • A holds displacement • e.g. segment registers in 80x86

  19. Indexed Addressing • Variables which use indexed addressing: • Variables referenced by pointers • Arrays • Local Variables, in stack-based languages. • E.g. (IX, IY registers) • A = base • R = displacement • EA = A + R • Automatically increments address with the value stored in an index register.

  20. Combinations • Combinations of the three basic addressing modes -> additional addressing modes. • Resultant or operational instruction cannot be immediate data. (Direct of Indirect memory location) • Postindex • EA = (A) + (R) • Preindex • EA = (A+(R))

  21. Stack Addressing • Data structure grows downward from high to low memory in SP. • New byte pushed onto the stack -> SP decremented • Byte pulled from the stack -> SP incremented • Operand is (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 Mode Calculation

  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

  25. PowerPC Memory Operand Addressing Modes

  26. Instruction Formats • Essentially the layout of the Bits in an Instruction set including: • OpCodes • Operand(s) expressed either implicitly or explicitly • Memory address • Register • Usually more than one instruction format is used and usually varies depending on whether its addressing Memory, I/O devices or a Register

  27. Instruction Length • Affected by and affects: • Memory size • Memory organization • Bus structure • CPU complexity • CPU speed

  28. Longer Instruction Length • Example: A 64 bit instruction is less than twice as useful as a 32 bit instruction. • Pros: • More OpCodes • Operands • Addressing Modes • Greater Address Range • Therefore easier Programming Language • Cons: • Waste of Space

  29. Allocation of Bits • Number of addressing modes: • i.e. certain opcodes always call for indexing, in others, addressing modes must be explicit and more than one bit might be needed. • Number of operands: • Typical instructions in today’s machines call for two operands. • Register versus memory: • The more registers that can be used for oprand references , the fewer bits are needed. Most resent day designs have at least 32 registers. • Number of register sets: • A method used by Pentium uses two sets of 8 registers and only 3 bits are required to identify the register. The opcode will determine which register is being referenced. • Address range: • Rarely used because it imposes a limitation in the amount of address since its directly related to the size of the address.

  30. PDP-8 Instruction Design • Only 12 bit Instructions • 7 Bits to Reference to Memory: • (2)^7 = 128 Different Words • 2 Bits used as modifiers ; • First referenced to page containing instruction • Second used to indicate whether direct or indirect addressing is to be used. • 3 Bits used for the opcode: • Of the 8 different opcode combinations, 2 are used to reference a page bit or an indirect leaving 6 basic operations.

  31. PDP-8 Instruction Format

  32. PDP-10 Instruction Format • Orthogonality • Principle that two variables are independent of each other. • Completeness • Each arithmetic data type (integer, fixed-point, real) should have a complete and identical set of operations. • Direct addressing • Used instead of base plus displacement addressing to lower the burden on the programmer.

  33. PDP-11 Instruction Format

  34. The PDP-11 was designed to provide a powerful and flexible instruction set • Only constraint was the use of a 16-bit minicomputer [BELL70] • It employs 16-bit general purpose registers • Two of these 16 have a significant purposes: • One is used as a stack pointer for special purpose stack operations • The other is used as the program counter (containing address of next instruction)

  35. VAX Instruction Examples

  36. VAX • VAX was designed to solve the problem of having a relatively small number of fixed instruction formats. - Having a small fixed instruction format posed two problems: • Addressing mode and opcode are not orthogonal • Only a limited number of operands can be accomodated • The solution: • All instructions should have the “natural” number of operands • All operands should have the same generality in specification.s

  37. Questions? • Why is it known immediate addressing? • Which addressing mode has a limited address space? • How does data structure grows in stack addressing? • Give two examples of indexed addressing modes. • Why do programmers prefer longer instruction ? • How many bits does the PDP-8 design use for addressing? • How is called the address the Pentium translation mechanism? • In register operand mode, what one has to with 64-bit floating point operation? • The PDP-10 was designed to be what kind of system and with what emphasis? • What two criteria were used in designing the VAX instruction format?

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