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The 8086/8088 Microprocessors: Registers

• Registers – Registers are in the CPU and are referred to by specific names – Data registers • Hold data for an operation to be performed • There are 4 data registers (AX, BX, CX, DX) – Address registers • Hold the address of an instruction or data element • Segment registers (CS, DS, ES, SS) • Pointer registers (SP, BP, IP) • Index registers (SI, DI) – Status register • Keeps the current status of the processor • On an IBM PC the status register is called the FLAGS register – In total there are fourteen 16-bit registers in an 8086/8088 Data Registers: AX, BX, CX, DX • Instructions execute faster if the data is in a register • AX, BX, CX, DX are the data registers • Low and High bytes of the data registers can be accessed separately – AH, BH, CH, DH are the high bytes – AL, BL, CL, and DL are the low bytes • Data Registers are general purpose registers but they also perform special functions • AX – Accumulator Register – Preferred register to use in arithmetic, logic and data transf

8086 internal registers 16 bits (2 bytes each)

8086 internal registers 16 bits (2 bytes each)

8086 internal registers 16 bits (2 bytes each)

8086 block Diagram

Intel 16-bit Registers

8086 Architecture • The 8086 has two parts, the Bus Interface Unit (BIU) and the Execution Unit (EU). • The BIU fetches instructions, reads and writes data, and computes the 20-bit address. • The EU decodes and executes the instructions using the 16-bit ALU. • The BIU contains the following registers: IP - the Instruction Pointer CS - the Code Segment Register DS - the Data Segment Register SS - the Stack Segment Register ES - the Extra Segment Register The BIU fetches instructions using the CS and IP, written CS:IP, to contr a ct the 20-bit address. Data is fetched using a segment register (usually the DS) and an effective address (EA) computed by the EU depending on the addressing mode. The EU contains the following 16-bit registers: AX - the Accumulator BX - the Base Register CX - the Count Register DX - the Data Register SP - the Stack Pointer

An Introduction to 8086 Microprocessor

Slide 2 Slide 2 8086 Features: • 16-bit Arithmetic Logic Unit • 20-bit address bus - 220 = 1,048,576 = 1 meg The address refers to a byte in memory. In the 8088, these bytes come in on the 8-bit data bus. In the 8086, bytes at even addresses come in on the low half of the data bus (bits 0-7) and bytes at odd addresses come in on the upper half of the data bus (bits 8-15). The 8086 can read a 16-bit word at an even address in one operation and at an odd address in two operations. The 8088 needs two operations in either case. The least significant byte of a word on an 8086 family microprocessor is at the lower address. Slide 3 Simplified CPU Design: