In this article, we will be discussing looping in AVR and branch instructions, both Conditional and Unconditional.
Looping in AVR :
A repeated operation or a set of instructions is known as a loop in programming. It is one of the most fundamental techniques which comes in very handy in writing code. One way to execute a loop in AVR is to write a set of instructions repeatedly.
Example -
For example:
LDI R20, 0
LDI R21, 1
ADD R20, R21
ADD R20, R21
ADD R20, R21
ADD R20, R21
ADD R20, R21
ADD R20, R21
As we can see, this way of executing a code is very inefficient and it takes up a lot of code space. Therefore, we use branch instructions (Conditional and Unconditional) to make the loop more simple and space-efficient.
Conditional Branch Instructions :
We have discussed the Conditional Branch Instructions in detail in
this article. We will discuss it in brief here.
The following table shows different conditional branch instructions along with their explanation.
| INSTRUCTION | EXPLANATION | FLAG STATUS |
|---|
| BREQ | Branch if equal | Branch if Z = 1 |
| BRNE | Branch if not equal | Branch if Z = 0 |
| BRSH | Branch if same or higher | Branch if C = 0 |
| BRLO | Branch if lower | Branch if C = 1 |
| BRLT | Branch if less than (signed) | Branch if S = 1 |
| BRGE | Branch if greater than or equal (signed) | Branch if S = 0 |
| BRVS | Branch if Overflow flag set | Branch if V = 1 |
| BRVC | Branch if Overflow flag clear | Branch if V = 0 |
Loop using BRNE :
The BRNE (branch if not equal) instruction uses the Z flag in the status register.
Example -
Write a program to add 5 to R20 20 times and send the sum to PORTC using the BRNE instruction.
LDI R16, 20; counter register
LDI R20, 0
LDI R21, 5
LOOP: ADD R20, R21
DEC R16; decrement the counter
BRNE LOOP; repeat until counter = 0
OUT PORTC, R20
All conditional branches are short jumps: This means that the address of the target must be within 64 bytes of the program counter.
Unconditional Branch Instructions :
The unconditional branch is a jump in which control is transferred unconditionally to the target address. In AVR, there are 3 unconditional branch instructions: JMP, RJMP, and IJMP. Using which instruction depends upon the target address.
- JMP (long jump) -
JMP is an unconditional jump that can go to any memory location in the 4M (word) address space of the AVR. It is a 4-byte instruction in which 10 bits are used for the opcode, and the other 22 bits represent the 22-bit address of the target location.
- RJMP (relative jump) -
In this 2-byte instruction, the first 4 bits are used for the opcode and the rest of the bits are used for the relative address of the target location. The relative address range of 000-$FFF is divided into forward and backward jumps, that is within -2048 to +2047 of memory relative to the address of the current program counter.
- IJMP (indirect jump) -
It is a 2-byte instruction. When it executes, the program counter is loaded with the contents of the Z register, so it jumps to the address provided by the Z register. IJMP can jump within the lowest 64K words of the program memory.
