ModMod Pengalamat

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Mod-Mod Pengalamat

• Segera (Immediate)
• Terus (Direct)
• Tak terus (Indirect)
• Daftar (Register)
• Daftar tak terus (Register indirect)
• Penggantian (Displacement - (indexed))

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Immediate Addressing

• Operan adalah sebahagian daripada arahan
• Operand address field
• e.g. ADD 5
• gt ADD 5 gt AC AC 5
• Tambah 5 kepada kandungan accumulator
• 5 adalah operan
• Tiada rujukan memori untuk mengambil data
• Pantas
• Julat yang terhad

Instruction
Operand
Opcode
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Direct Addressing

• Medan alamat (address field) mengandungi alamat
  bagi operan
• Alamat efektif alamat medan
• (Effective address (EA) address field (A))
• Cth ADD A
• Tambah kandungan bagi sel A kepada penumpuk
  (accumulator)
• Cari operan dalam memori pada alamat A
• Rujukan memori tunggal untuk mengakses data
• Tiada tambahan congakkan untuk mendapatkan EA
• Ruang alamat terhad

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Direct Addressing Diagram
Instruction
Address A
Opcode
Memory
Operand
gt Load a gt ac lt a
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Indirect Addressing (1)

• Memory cell 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

gt Add a, (b) gt a a (b)
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Indirect Addressing (2)

• Large address space
• 2n where n word length
• May be nested, multilevel, cascaded
• e.g. EA (((A)))
• Multiple memory accesses to find operand
• Hence slower

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Indirect Addressing Diagram
Instruction
Address A
Opcode
Memory
Pointer to operand
Operand
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Register Addressing (1)

• Operand is held in register named in address
  filed
• EA R
• gt ADD R1, R2
• gt R1 r1 r2
• Limited number of registers
• Very small address field needed
• Shorter instructions
• Faster instruction fetch

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Register Addressing (2)

• No memory 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

cf. is an abbreviation from a Latin word (confer)
and means 'refer to' or 'compare with' N.B
nota bene - Used to direct attention to something
particularly important
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Register Addressing Diagram
Instruction
Register Address R
Opcode
Registers
Operand
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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

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

• EA A (R)
• Address field hold two values
• A base value
• R register that holds displacement
• or vice versa

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

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Relative Addressing

• A version 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

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Base-Register Addressing

• A holds displacement
• R holds pointer to base address
• R may be explicit or implicit
• e.g. segment registers in 80x86

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Indexed Addressing

• A base
• R displacement
• EA A R
• Good for accessing arrays
• EA A R
• R

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Combinations

• Postindex
• EA (A) (R)
• Preindex
• EA (A(R))

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Stack Addressing

• Operand is (implicitly) on top of stack
• e.g.
• ADD Pop top two items from stack and add