Addressing

Another Form of Storage:

Along with main storage, there are also 16 General Purpose Registers (GPRs).

Accessed by the computer faster than main storage
Hold 32 bits
Numbered from 0 to 15

Addressing Main Storage:

Every byte of storage has an absolute address, ranging from 000000 to FFFFFF. The absolute address is the address of the first byte of an instruction or data item that is referred to.

A slight problem: a programmer does not know where his/her program will be stored in main storage.

To solve the problem: use base-displacement, relative or explicit addressing. The idea behind this is that the position of two items is fixed. If the absolute address of one is known, then the address of the other statement can be figured out by calculating the displacement between the two items.

Relative address = base address + displacement

The base address is placed in a GPR when a program is executed. The GPR is called a base register.

Two forms of relative addressing:

D(B)
- D is the displacement. Decimal number between 0 - 4095
- B is the base register number. GPR between 0 - 15
4(5) read "4 off of register 5"

36(7)

How it works: Convert the displacement to hex and then add it to the LAST three bytes of the address in the register. 1^st byte is ignored.

Assume register 7 holds 010234A0, then:
```
  36(7)  =  010234A0
          +       24
              0234C4
  
```
Special Case: If the register number is 0, the absolute address 00000000 is used in the calculation.

If register 0 holds 2A763598, then 36(0) results in 000024.

D(X,B)
- D is the displacement. Decimal number between 0 - 4095
- X is the index register number. GPR between 0 - 15
- B is the base register number. GPR between 0 - 15
4(5,7)

8(9) -- 9 is the index register

2(,3) -- 3 is the base register

Register number defaults to 0 if left off.

Assume register 4 holds 0500029B and register 7 holds 000027A4, then:
```
  3(4,7)  =  000027A4
             0500029B
            +       3
               002A42
  
```