General Purpose Registers

eax - ax - ah/al
ebx - bx - bh/bl
ecx - cx - ch/cl
edx - df - dh/dl
esp - sp : stack pointer
ebp - bp : base pointer (points to stack base)
edi - di : destination index (used for stream operations)
esi - si : source index (used for stream operations)

Segment Registers

ss - stack segment
cs - code segment
ds - data segment
es - extra segment
fs - extra extra segment
gs - extra extra extra segment

Memory Segmentation

In real mode we can address ‘65,535’ bytes into a segment using a 16-bit offset. A memory address is calculated by shifting the segment address four bits to the left and then adding the offset bits which cleverly gives us 20 bits of address space on a 16-bit system.

E.g. given a segment address of ‘0xd0d0’ and an offset of ‘0x00fa’ : shift the segment four bits left ( ‘0xd0d00’ ), add the offset ( ‘0xd0d00 + 0xb14d’ ), and the calculated address is ‘0xdbe4d’

Segments begin at 16 byte intervals and since we can address into segments with ‘65,535’ byte offsets there is a *lot* of segment overlap.

Another layer of complexity here is that if we start with our last segment ( ‘0xffff’ ) and shift it four bits left we get ‘0xffff0’ which is at the very top of our 20-bit address space (with just 16 bytes to spare), so this actually gives us the ability to address ‘65,519’ ( ‘65535 - 16’ ) bytes above our 20-bit address space.

Due to the overlapping segments there are a significant number of segment/offset combinations to access a single memory address.

Real Mode Addressing Modes

BX + val
SI + val
DI + val
BP + val
BX + SI + val
BX + DI + val
BP + SI + val
BP + DI + val
address

Code Page 437