AMD AMD-K6-2/400 User Guide - Page 50
General-Purpose, Registers, Software Environment,
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Preliminary Information AMD-K6™-2E+ Embedded Processor Data Sheet 23542A/0-September 2000 General-Purpose Registers 31 The eight 32-bit x86 general-purpose registers are used to hold integer data or memory pointers used by instructions. Table 2 contains a list of the general-purpose registers and the functions for which they are used. Table 2. General-Purpose Registers Register Function EAX Commonly used as an accumulator EBX Commonly used as a pointer ECX Commonly used for counting in loop operations EDX Commonly used to hold I/O information and to pass parameters EDI Commonly used as a destination pointer by the ES segment ESI Commonly used as a source pointer by the DS segment ESP Used to point to the stack segment EBP Used to point to data within the stack segment In order to support byte and word operations, EAX, EBX, ECX, and EDX can also be used as 8-bit and 16-bit registers. The shorter registers are overlaid on the longer ones. For example, the name of the 16-bit version of EAX is AX (low 16 bits of EAX) and the 8-bit names for AX are AH (high order bits) and AL (low order bits). The same naming convention applies to EBX, ECX, and EDX. EDI, ESI, ESP, and EBP can be used as smaller 16-bit registers called DI, SI, SP, and BP respectively, but these registers do not have 8-bit versions. Figure 7 shows the EAX register with its name components, and Table 3 on page 29 lists the doubleword (32-bit) general-purpose registers and their corresponding word (16-bit) and byte (8-bit) versions. 16 15 87 0 EAX AX AH AL Figure 7. EAX Register with 16-Bit and 8-Bit Name Components 28 Software Environment Chapter 3