Assembly Language and Computer Structure

Study Notes

Assembly language is a low-level programming language that requires knowledge of computer structure to understand.
The simple computer model consists of a system bus, CPU, and RAM, where the CPU is the heart of the computer and performs most computations.

The 8086 CPU has 8 general purpose registers, each with its own name and 16-bit size.
The registers are: AX, BX, CX, DX, SI, DI, BP, and SP.
Despite their names, the programmer determines the usage for each register.
The main purpose of a register is to keep a number (variable).
4 general purpose registers (AX, BX, CX, DX) are made of two separate 8-bit registers (e.g., AH and AL).
Modifying an 8-bit register updates the corresponding 16-bit register, and vice-versa.
Registers are faster than memory because they are located inside the CPU and do not require the use of a system bus.

The segment registers are: CS, DS, ES, and SS.
CS points to the segment containing the current program.
DS generally points to the segment where variables are defined.
ES is an extra segment register, and its usage is defined by the coder.
SS points to the segment containing the stack.
Segment registers work together with general purpose registers to access any memory value.
The CPU calculates the physical address by multiplying the segment register by 10h and adding the general purpose register to it.

The address formed with 2 registers is called an effective address.
By default, BX, SI, and DI registers work with the DS segment register, and BP and SP work with the SS segment register.
Other general purpose registers cannot form an effective address.
IP (instruction pointer) and flags register are special purpose registers.
IP register always works together with the CS segment register and points to the currently executing instruction.
Flags register is modified automatically by the CPU after mathematical operations and determines the type of the result.