Tutorial by Examples

When Intel defined the original 8086, it was a 16-bit processor with a 20-bit address bus (see below). They defined 8 general-purpose 16-bit registers - but gave them specific roles for certain instructions: AX The Accumulator register. Many opcodes either assumed this register, or were faster i...

When Intel produced the 80386, they upgraded from a 16-bit processor to a 32-bit one. 32-bit processing means two things: both the data being manipulated was 32-bit, and the memory addresses being accessed were 32-bit. To do this, but still remain compatible with their earlier processors, they intro...

The first four 16-bit registers could have their upper- and lower-half bytes accessed directly as their own registers: AH and AL are the High and Low halves of the AX register. BH and BL are the High and Low halves of the BX register. CH and CL are the High and Low halves of the CX register. D...

Segmentation When Intel was designing the original 8086, there were already a number of 8-bit processors that had 16-bit capabilities - but they wanted to produce a true 16-bit processor. They also wanted to produce something better and more capable than what was already out there, so they wanted t...

AMD is a processor manufacturer that had licensed the design of the 80386 from Intel to produce compatible - but competing - versions. They made internal changes to the design to improve throughput or other enhancements to the design, while still being able to execute the same programs. To one-up I...

When the x86 Arithmetic Logic Unit (ALU) performs operations like NOT and ADD, it flags the results of these operations ("became zero", "overflowed", "became negative") in a special 16-bit FLAGS register. 32-bit processors upgraded this to 32 bits and called it EFLAGS, ...