Describe the execution unit (EU) of 8086 microprocessor.

EU (Execution Unit): Execution unit gives instructions to BIU stating from where to fetch the data and then decode and execute those instructions. Its function is to control operations on data using the instruction decoder & ALU. EU has no direct connection with system buses as shown in the above figure, it performs operations over data through BIU.

Let us now discuss the functional parts of 8086 microprocessors.

ALU: It handles all arithmetic and logical operations, like +, -, X, /, OR, AND, NOT operations.

Flag Register: It is a 16 bit register that behaves like a flip flop, i.e. it changes its status according to the result stored in the accumulator. It has 9 flags and they are divided into 2 groups - Conditional Flags and Control Flags.

Conditional Flags: It represents the result of the last arithmetic or logical instruction executed. Following is the list of conditional flags -

(i) Carry flag: This flag indicates an overflow condition for arithmetic operations.

(ii) Auxiliary flag: When an operation is performed at ALU, it results in a carry/barrow from lower nibble (i.e. D0 - D3) to upper nibble (i.e. D4 - D7), then this flag is set, i.e. carry given by D3 bit to D4 is AF flag. The processor uses this flag to perform binary to BCD conversion.

(iii) Parity flag: This flag is used to indicate the parity of the result, i.e. when the lower order 8-bits of the result contains even number of 1’s, then the Parity Flag is set. For odd number of 1’s, the Parity Flag is reset.

(iv) Zero flag: This flag is set to 1 when the result of arithmetic or logical operation is zero else it is set to 0.

(v) Sign flag: This flag holds the sign of the result, i.e. when the result of the operation is negative, then the sign flag is set to 1 else set to 0.

(vi) Overflow flag: This flag represents the result when the system capacity is exceeded.

Control Flags: Control flags controls the operations of the execution unit. Following is the list of control flags -

(i) Trap flag: It is used for single step control and allows the user to execute one instruction at a time for debugging. If it is set, then the program can be run in a single step mode.

(ii) Interrupt flag: It is an interrupt enable/disable flag, i.e. used to allow/prohibit the interruption of a program. It is set to 1 for interrupt enabled condition and set to 0 for interrupt disabled condition.

(iii) Direction flag: It is used in string operation. As the name suggests when it is set then string bytes are accessed from the higher memory address to the lower memory address and vice-a-versa.

General purpose register: There are 8 general purpose registers, i.e. AH, AL, BH, BL, CH, CL, DH, and DL. These registers can be used individually to store 8-bit data and can be used in pairs to store 16-bit data. The valid register pairs are AH and AL, BH and BL, CH and CL, and DH and DL. It is referred to the AX, BX, CX, and DX respectively.

(i) AX register: It is also known as accumulator register. It is used to store operands for arithmetic operations.

(ii) BX register: It is used as a base register. It is used to store the starting base address of the memory area within the data segment.

(iii) CK register: It is referred to as counter. It is used in loop instruction to store the loop counter.

(iv) DX register: This register is used to hold I/O port address for I/O instruction.

Stack pointer register: It is a 16-bit register, which holds the address from the start of the segment to the memory location, where a word was most recently stored on the stack.