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Flag register of 8086 microprocessor
The flag register is one of the special purpose register. The flag bits are changed to 0 or 1 depending upon the value of result after arithmetic or logical operations.
8086 has 16-bit flag register, and there are 9 valid flag bits. The format of flag register is like below.
| Bits | D15 | D14 | D13 | D12 | D11 | D10 | D9 | D8 | D7 | D6 | D5 | D4 | D3 | D2 | D1 | D0 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Flags | O | D | I | T | S | Z | AC | P | CY |
We can divide the flag bits into two sections. The Status Flags, and the Control Flags.
Status Flags
In 8086 there are 6 different flags which are set or reset after 8-bit or 16-bit operations. These flags and their functions are listed below.
| Flag Bit | Function |
|---|---|
| S | After any operation if the MSB is 1, then it indicates that the number is negative. And this flag is set to 1 |
| Z | If the total register is zero, then only the Z flag is set |
| AC | When some arithmetic operations generates carry after the lower half and sends it to upper half, the AC will be 1 |
| P | This is even parity flag. When result has even number of 1, it will be set to 1, otherwise 0 for odd number of 1s |
| CY | This is carry bit. If some operations are generating carry after the operation this flag is set to 1 |
| O | The overflow flag is set to 1 when the result of a signed operation is too large to fit. |
Control Flags
In 8086 there are 3 different flags which are used to enable or disable some basic operations of the microprocessor. These flags and their functions are listed below.
| Flag Bit | Function |
|---|---|
| D | This is directional flag. This is used in string related operations. D = 1, then the string will be accessed from higher memory address to lower memory address, and if D = 0, it will do the reverse. |
| I | This is interrupt flag. If I = 1, then MPU will recognize the interrupts from peripherals. For I = 0, the interrupts will be ignored |
| T | This trap flag is used for on-chip debugging. When T = 1, it will work in a single step mode. After each instruction, one internal interrupt is generated. It helps to execute some program instruction by instruction. |
Updated on: 06-Sep-2023
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