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Contents > General-purpose Computer > Assembly Language
A branch instruction redirects execution to a specified location, either conditionally or unconditionally. With one exception, the branch instructions have the following format.
mnemonic label
A label is a placeholder for an address. In this case, it is the address of the code to run if the branch is taken.
A label declaration consists of a unique name followed by a colon, as shown below.
name:
The declaration appears immediately before target code or data, and the assembler resolves its address based on its position.
Here are all the branch instructions:
| Instruction | Name | Pseudocode | Opcode |
|---|---|---|---|
| JMP label | Jump | P = label; | 20 |
| BNE label | Branch Not Equal | if (z == 0) P = label; | 22 |
| BEQ label | Branch Equal | if (z == 1) P = label; | 23 |
| BPL label | Branch Plus | if (n == 0) P = label; | 24 |
| BMI label | Branch Minus | if (n == 1) P = label; | 25 |
| JSR label | Jump Subroutine | R = P; P = label; | 28 |
| RTS | Return Subroutine | P = R; | 70 |
RTS is a 1-byte instruction. The rest are 3-byte instructions consisting of an opcode followed by a big-endian address. The opcode comprises bits 0010rstu, where only one of r, s, and t are set, and u is a value compared to s or t:
| Set Bit | Instructions | Description |
|---|---|---|
| r | JSR | The computer copies the program counter, P, to the return register, R, and it branches. |
| s | BPL,BMI | The computer branches iff the negative flag, n, equals bit u. |
| t | BNE,BEQ | The computer branches iff the zero flag, z, equals bit u. |
JSR supports only leaf subroutines since the computer does not provide a native call stack.
A program can use the conditional branch instructions as relational operators by branching on the result of A - B. However, BPL and BMI treat the difference as a signed number. This leads to the following limitations.
| Instruction | Relation | Limitations |
|---|---|---|
| BNE | A ≠ B | none |
| BEQ | A = B | none |
| BPL | A ≥ B | A - B ∊ [0, 127] |
| BMI | A < B | A - B ∊ [-128, -1] |
While there is no indirect branch instruction, a program can employ self-modifying code to alter a branch target at runtime.
By default, the computer initializes all registers to 0 at startup. However, a program can declare an entry point with a label named main. If present, the computer initializes P to its address.
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