ECE 362 - Microcontroller design

Module 1

Addressing modes

1. Immediate Addressing Mode
   • programmer provides value to be acted upon
     
   • This value is hard-coded and not read from memory
     
   • eg. movl $1,%eax
     • move value "1" into register eax
2. Direct Addressing Mode
   • The address of mentioned location/variable is acted upon
   • eg. movl $data1,%ecx
     • address of variable data1 is stored into register ecx
3. Register Addressing Mode
   • Moving data in and out of a register
   • Since eax and ecx are registers, above examples hold true
4. Indexed Addressing Mode

   .data
   data1 10,11,22,33,44,55,66,77,88,99
   ...
   _start:
   movl $0,%edi ; direct addressing here. Index register
   movl data(,%edi,4),%eax
   ; movl addr_or_offset(%base_or_offset,%index,multiplier)
   ; index : index location
   ; multiplier : space taken by each block of memory
   ;              eg: long takes 4 blocks
   ; IF %edi was some other value, that location would be
   ;    extracted and the content be saved into eax

5. Indirect Addressing Mode
   • Store memory address in register
   • Access register to get address to read that location to get value
   • example still due

6. Base Pointer Addressing Mode

   • Given a pointer to an address, Can get data at pointed location and other nearby locations by using offsets

   pushl $7
   pushl $8
   pushl $77
   pushl %ebp ; save value of base ptr to stack, now can overwrite
   movl %esp,%ebp ; over-write base ptr with stack ptr
    
   movl 4(%ebp),%eax ; save 4+esp (77) to eax register
   movl 8(%ebp),%eax  ;
   movl 12(%ebp),%eax ; base pointer addressing

Addressing Modes (IIT-M)

Consider having an operation: ADD X,Y [opcode | source operand | destionation operand] [opcode | (mode,register) | (mode,register) ] (mode : ~16 codes) (register : ~8 registers)

We need a way to address the data points X and Y

1. Register
   • The register itself holds the data
2. Direct Addressing Mode
   • The register holds address of a location that contains payload
   • (Needs one machine cycle - puts data on bus once)

3. Indirect (Deffered) Addressing Mode

   • The register holds the address of a location that then holds the address of another location which contains payload

     Consider Register in CPU to have address "1000"
     In the Memory, let us have locations numbers 1000 to 1200
     Let value in address "1000" be the address "1010"
     Here, register points to 1000 in memory which points to 1010 in memory, which in turn contains the final payload.

   • (Needs 2 machine cycles)

4. Index Mode
   • An index Value / Displacement, say 'X' is provided
   • The Register address is added with 'X' to arrive to the Final/Expected address
   • Register is called Base register
5. Index Deffered Mode
   • Here, the Final?Expected Address points to another final address
6. Auto-(Increment|Decrement) Mode
   • Register holds address
   • After the data required is extracted, the register value is incremented or decremented
7. Immediate Mode
   • Here the address is available as part of the instruction
   • No need to use the register
8. Relative Addressing
   • Usually, program counter is used instead of register
   • Relative allows you to see how far away in either direction of base