Addressing modes & Flag Bits

Five possible addressing modes plus the combinations are as follows.

Direct (x, b, and p all set to 0): operand address goes as it is. n and i are both set to the same value, either 0 or 1. While in general that value is 1, if set to 0 for format 3 we can assume that the rest of the flags (x, b, p, and e) are used
as a part of the address of the operand, to make the format compatible to the
SIC format

Relative (either b or p equal to 1 and the other one to 0): the address of the operand should be added to the current value stored at the B register (if b = 1) or to the value stored at the PC register (if p = 1)

Immediate (i = 1, n = 0): The operand value is already enclosed on the instruction (ie. lies on the last 12/20 bits of the instruction)

Indirect (i = 0, n = 1): The operand value points to an address that holds the address for the operand value.

Indexed (x = 1): value to be added to the value stored at the register x to obtain real address of the operand. This can be combined with any of the previous modes except immediate.

The various flag bits used in the above formats have the following meanings

e - e = 0 means format 3, e = 1 means format 4

Bits x,b,p: Used to calculate the target address using relative, direct, and indexed addressing Modes

Bits i and n: Says, how to use the target address

b and p - both set to 0, disp field from format 3 instruction is taken to be the target address. For a format 4 bits b and p are normally set to 0, 20 bit address is the target address

x - x is set to 1, X register value is added for target address calculation
i=1, n=0 Immediate addressing, TA: TA is used as the operand value, no memory reference
i=0, n=1 Indirect addressing, ((TA)): The word at the TA is fetched. Value of TA is taken as the address of the operand value

i=0, n=0 or i=1, n=1 Simple addressing, (TA):TA is taken as the address of the operand value
Two new relative addressing modes are available for use with instructions assembled using format 3.

Mode Indication Target address calculation
Base relative b=1,p=0 TA=(B)+ disp
(0£disp £4095)
Program-counter relative b=0,p=1 TA=(PC)+ disp
(-2048£disp £2047)