CPU Registers

CPU registers are classified into five categories as follows

1. Segment registers
2. Pointer registers
3. General Purpose registers
4. Index registers
5. Flags register

1. Segment registers

• Segments (20-bit wide) are special areas defined in a program for containing the code, the data and stack.
• segment begins on a paragraph boundary; that is at a location evenly divisible by 16
• segment registers are 16-bit size and contains starting address of the segment (Reason: since segments are starting on a paragraph boundary, the designers decided that it would be unnecessary to store the zero digit in the segment register)
• Offset is 16 bits wide (and is specified in Pointer registers described later)
• Further classified into Code, data, stack and extra - corresponds to CS, DS, SS, ES, FS and GS registers

2. Pointer registers

• Pointer registers are 32-bit EIP, ESP and EBP; the rightmost are IP, SP and BP respectively (16-bit wider as mentioned above)
• IP register is associated with CS register (as CS:IP => Segment:Offset)

Example - Segment address in CS 39B40h

Offset address in IP +0514h

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Address of next instruction 3A054h

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• SP register is associated with SS register (as SS:SP => Segment:Offset)

Example - Segment address in SS 39B40h

Offset address in SP +0514h

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Address in stack 3A054h

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• BP facilitates referencing parameters, which are data and addresses that a program passes via the stack. Processor combines the address in SS with the offset in BP. BP can also be combined with DI and with SI as a base register for special addressing.

3. General Purpose registers

• 32-bit general purpose registers
• AX - primary accumulator - used for operations involving input/output and most arithmetic - more efficient compared to other registers
• BX - base register - only register used as an index to extend addressing - can also be combined with DI or SI as a base register for special addressing
• CX - count register - may contain a value to control the number of times a loop is repeated or a value to shift bits left or right
• DX - data regsiter - works with AX sometimes, to compute operations that involve large values

4. Index registers

• SI (soure index) - may be required for some string (character) handling operations - in this context, SI is associated with DS register (as DS:SI)
• DI (destination index) - is required for some string operations - in this context, DI is associated with ES register

5. Flags register

• 32 bit wder
• OF (overflow), IF (interrupt), TF (trap), SF (sign), ZF (zero), AF (auxiliary carry), PF (parity) and CF (carry)