Low byte and high byte in a frame

Low byte and high byte in a frame

The byte is a unit that most commonly consists of eight bits. A byte is the number of bits used to encode the character of text in a computer, therefore, for this reason it is the basic addressable element in many computer architectures.

Modern (ie, 386 and above) x86 microprocessors have eight 32-bit  general-purpose registers, as shown in Figure A. These register names are mostly historical. For example, EAX called as Accumulator, since it was used by a number of arithmetic operations, and ECX  known as the counter since it was used to hold a loop index. While most of the registers have lost their special purposes in modern instruction set, by convention, two are reserved for special purposes – the stack pointer (ESP) and the base pointer (EBP).For the EAX,EDX , EBX, ECX registers, subsections may be used. For example, the least significant 2 bytes of EAX can be treated as a 16-bit register called AX. The least significant byte of AX can be used as a single 8-bit register called Al, while the most significant byte of AX can be used as a single 8-bit register called AH. These names refer to the physical register. When a two-byte number(Quantity) is placed in DX, the update affects the value of DH and DL, and EDX. These sub-registers are mainly hold-overs from older, 16-bit versions of the instruction set. However, sometimes they are convenient when it comes to data that are smaller than 32-bits (eg 1-byte ASCII characters).