Conditional Branch Instructions in AVR Microcontrollers: Controlling Program Flow
Master conditional branch instructions in AVR assembly language programming. This guide explains how these instructions, based on status register flags, enable the implementation of loops, conditional statements, and efficient control flow in AVR microcontroller programs.
Conditional Branch Instructions in AVR Microcontrollers
Controlling Program Flow
Controlling a program's flow is essential in any programming language. In assembly language programming for AVR microcontrollers, this is achieved using branch instructions—instructions that cause the program to jump to a different location in memory instead of executing instructions sequentially.
The Status Register (SREG)
The AVR microcontroller's status register (SREG), also called the flag register or condition code register, stores information about the result of previous instructions. This 8-bit register contains several flags, including:
| Bit | Flag | Description |
|---|---|---|
| 0 | C (Carry) | Set if there's a carry out of the most significant bit (MSB) in an arithmetic operation. |
| 1 | Z (Zero) | Set if the result of an arithmetic operation is zero. |
| 2 | N (Negative) | Set if the MSB of the result is 1 (indicating a negative number in two's complement). |
| 3 | V (Two's Complement Overflow) | Set if an arithmetic operation results in an overflow. |
| 4 | S (Sign) | Set if the MSB of the result is 1. (Same as the Negative flag.) |
| 5 | H (Half Carry) | Set if there's a carry from bit 3 to bit 4 during addition or subtraction. |
| 6 | T (Bit Copy Storage) | Used for bit manipulation instructions. |
| 7 | I (Global Interrupt Enable) | Enables or disables interrupts. |
Conditional Branch Instructions
Conditional branch instructions check the status register flags and jump to a different instruction if a specific condition is met.
| Instruction | Mnemonic | Condition | Branch Condition |
|---|---|---|---|
| Branch if Equal | BREQ | Z (Zero Flag) | Branch if Z = 1 |
| Branch if Not Equal | BRNE | Z (Zero Flag) | Branch if Z = 0 |
| Branch if Same or Higher (Unsigned) | BRSH | C (Carry Flag) | Branch if C = 0 |
| Branch if Lower (Unsigned) | BRLO | C (Carry Flag) | Branch if C = 1 |
| Branch if Less Than (Signed) | BRLT | S (Sign Flag) | Branch if S = 1 |
| Branch if Greater Than or Equal (Signed) | BRGE | S (Sign Flag) | Branch if S = 0 |
| Branch if Overflow Set | BRVS | V (Overflow Flag) | Branch if V = 1 |
| Branch if Overflow Cleared | BRVC | V (Overflow Flag) | Branch if V = 0 |
(Note that the branch instructions are relative; the jump target is specified as an offset from the program counter.)
Conclusion
Conditional branch instructions are essential for creating flexible and responsive programs. They allow the program's execution flow to adapt based on the results of computations, enabling decision-making within the program. The status register flags provide the information needed for these conditional branches.