Introduction to Program segments

Main memory, which is 1M bytes, is divided into segments. Each segment size is 64K bytes. In 8086/8088 architecture, segmented memory model is used. The reason for using segmented memory model is to make it compatible with earlier architectures 8085/8080 which had total 64K bytes main memory. Codes/Programs written for 8085/8080 architectures will run on architecture 8086/8088.

The main memory being 1M bytes has physical address of 20-bits. To access data from physical memory location, processor generates 20-bit address. This address is generated using segment and offset registers. Steps are as follows;

1. Shift left segment value by 4-bits (binary).

2. Shift right offset by 4-bits

3. Adds these two numbers,

4. Result produced is 20-bit physical address.

Example

If you specify segment value as CS=0A00 and offset value as IP=1234; Processor will read/write data from Physical address calculated as below;

CS= 0000 1010 0000 0000 - - - - (shifted left by 4-bits)

IP = - - - - 0001 0010 0011 0100

Physical address =0000 1011 0010 0011 0100

Code segment

When a code is assembled, the executable part (instructions) are placed in code segment and are accessed one by one. Instruction pointer (IP) is a register that always points to next executable instruction in code segment. It holds the address of next instruction which is to be executed. To read instructions manually from any part of code segment, address can be specified using segment and offset. Segment has to be specified via segment register CS and offset can be given using immediate value or instruction pointer (IP).

For example, to access instruction from memory address 0Xabcde; CS can be initialized as CS=A000 and offset can be chosen as offset=BCDE;

Data segment

The variables or arrays declared in a program are stored in data segment. Processor generates physical memory address to read/write data from data segment in similar way as explained above with the only difference that registers used to access data are different.

Whenever you want to access a memory variable, you can specify its physical address by specifying its segment address in DS offset address in either of (Si, Di or BX) registers.

Stack segment

Variables created within functions/procedures, parameters needed in a function call, return addresses of functions & interrupts are always saved in stack memory. To access data from stack segment, Stack Segment register (SS) is used as segment register and base pointer (BP) is used as offset register.

Extra segment

Part of memory which is used to temporary store memory variables. Extra segment register (ES) can be used to access data from other segments too. Usually used in string operations.