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 transfer instructions because it generates the shortest Machine Language Code
– Must be used in multiplication and division operations
– Must also be used in I/O operations