Instruction Sets 3

1
Instruction Sets 3

• Lecture 10
• Ref Ch 11

2
Outline

• Review
• Addressing Modes
• Instruction Formats
• Pentium vs PowerPC

3
Immediate Addressing Diagram
Instruction
Operand
Opcode

• ADD 5
• Add 5 to contents of accumulator
• 5 is operand

4
Direct Addressing Diagram
Instruction
Memory
Address A
Opcode
Operand

• ADD A
• Add contents of cell A to accumulator
• Look in memory at address A for operand

5
Indirect Addressing

• Large address 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

6
Indirect Addressing Diagram
Instruction
Address A
Opcode
Memory
Pointer to operand
Operand

• ADD (A)

7
Register Addressing

• EA R
• No memory access
• Very fast execution
• Very limited address space
• Multiple registers helps performance
• Requires good assembly programming or compiler
  writing

8
Register Addressing Diagram
Instruction
Register Address R
Opcode
9
Register 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

10
Register Indirect Addressing Diagram EA (R)
Instruction
Register Address R
Opcode
Memory
Registers
Operand
Pointer to Operand
11
Displacement Addressing

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

12
Displacement Addressing Diagram EA A (R)
13
Displacement Addressing -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

14
Displacement Addressing - Base-Register Addressing

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

15
Displacement Addressing - Indexed Addressing

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

16
Stack Addressing

• Operand is (implicitly) on top of stack
• e.g.
• ADD Pop top two items from stack and add
• Pentium vs PowerPC Addressing Modes - See Tables
  11.2 and 11.3

17
Instruction Length

• Affected by and affects
• Memory size
• Memory organization
• Bus structure
• CPU complexity
• CPU speed
• Trade off between powerful instruction repertoire
  and saving space

18
Allocation of Bits

• Number of addressing modes
• Number of operands
• Register versus memory
• Number of register sets
• Address range
• Address granularity
• Examples Figures 11.8 (Pentium) and 11.9
  (PowerPC)

19
Reading

• Capt /Sect 12.1-12.3