Machine Architecture: 2.2 8086 Architecture Overview: Registers

GEOS SDK TechDocs

2.1 Memory

2.3 The Prefetch Queue

The Intel 80x86 processors have thirteen registers plus one register containing machine status flags. The thirteen registers are separated into four logical groups by their use (they are all 16 bits):

• Instruction Pointer
  This single register maintains the address of the current instruction being executed. This is not accessed by applications.
• Segment Registers
  These four registers contain segment pointers.
• Index and Pointer Registers
  These four registers contain offsets into segments.
• General registers
  These four registers can contain any general data. They may be operated on as words or bytes.

The Segment and Index registers are used in conjunction to access memory. A program may have four segments pointed to at once: A code segment (CS), a data segment (DS), a stack segment (SS), and an extra segment (ES). These segments have various uses and purposes described in most 8088/8086 books.

The four Index registers are used as offsets into the segments pointed to in the Segment registers. They are the Stack Pointer (SP), the Base Pointer (BP), the Source Index (SI), and the Destination Index (DI). Each of these index registers has special applications with certain instructions.

The General registers are four 16-bit registers that may be used for any purpose. However, due to the early restrictions of the 8080 that carried over into the later processors, some instructions place their results or take their source data from specific registers.

All four general registers may be accessed as a word or as two separate bytes. The four registers are AX (the accumulator), BX (the base register), CX (the counter), and DX (the data register). The high byte of any of these may be accessed by substituting "H" for "X," and the low byte may be accessed by substituting "L" for "X." (The "H" stands for "high" and the "L" for "low.")

GEOS SDK TechDocs

2.1 Memory

2.3 The Prefetch Queue