Addressing Modes
Immediate Addressing
Operand is part of instruction
Operand = address field
No memory reference to fetch data
Fast but limited range
Immediate Addressing Diagram 
Direct Addressing
Address field contains address of operand
Effective address (EA) = address field (A)
Single memory reference to access data
No additional calculations to work out
effective address
Limited Address Space
Direct Addressing Diagram
Indirect Addressing
Memory cell pointed to by address field contains the address of (pointer to) the operand
Multiple memory accesses to find operand, Hence slower
Register Addressing
Limited number of registers
No memory access
Very fast execution
Register Indirect Addressing
Operand is in memory cell pointed to by
contents of register R
One fewer memory access than indirect
addressing
Displacement Addressing Diagram
Relative
Base-register
Indexing
Relative Addressing
get operand from A cells from current
location pointed to by PC (program counter)
Base-Register Addressing
Very similar to "Relative Addressing"except that any register is used instead of PC
Indexed Address
Combinations
Post-index
Pre-index
EA = (A) + (R)
EA = (A+(R))
Stack Addressing
Operand is (implicitly) on top of stack
Addressing Modes
x86 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
ARM Addressing Modes Load/Store
Offset
Preindex
Postindex
added to or subtracted from base register contents to form
the memory address
++i
i++
ARM
Data Processing
Branch
Multiple Addressing
Register addressing
Immediate
Instruction contains 24 bit value
Instructions
Formats
Usually more than one instruction format
in an instruction set
Length
Memory size
Bus structure
CPU complexity
CPU speed
Memory organization
Allocation of Bits
Number of addressing modes
Number of register sets
Number of operands
PDP-8
PDP-10
PDP-11
x86
ARM
Thumb Instruction Set
Re-encoded subset of ARM instruction set
Increases performance in 16-bit or less
data bus
Assembler
Machines store and understand binary
instructions
Improvements
Use hexadecimal rather than binary
Symbolic Addresses
Now have assembly language and need an
assembler to translate