Branch
The branch instructions perform a test against one of the processor's flags. Depending on the instruction a branch is taken if it is either clear or set.
If the branch is taken, a 1-byte signed displacement in the second byte of the instruction is sign-extended to 16-bits and added to the Program Counter. If the branch is not taken then the instruction immediately following the 2-byte instruction is executed.
The allowable range of the displacement is -128 to +127 from the instruction immediately following the branch.
BCC - Branch if Carry Clear
BCC tests the Carry flag and branches if it is clear.
It can be used in several ways:
- Test the result of a shift into the carry
- Determine if the result of a comparison is less than
Some assemblers accept BLT (Branch if Less Than) as an alternate mnemonic for BCC.
BCS - Branch if Carry Set
BCS tests the Carry flag and branches if it is set.
It can be used in several ways:
- Test the result of a shift into the carry
- Determine if the result of a comparison is greater than or equal
Some assemblers accept BGE (Branch if Greater Than or Equal) as an alternate mnemonic for BCS.
BEQ - Branch if Equal
BEQ tests the Zero flag and branches if it is set.
It can be used in several ways:
- Test the result of a comparison is equal
- Test the result of an Increment or Decrement operation is zero, useful in loops.
- Test the value just loaded is zero
- Test the result of an arithmetic operation is zero
BNE - Branch if Not Equal
BNE tests the Zero flag and branches if it is clear.
It can be used in several ways:
- Test the result of a comparison is not equal
- Test the result of an Increment or Decrement operation is not zero
- Test the value just loaded is not zero
- Test the result of an arithmetic operation is not zero
BMI - Branch if Minus
BMI tests the Negative flag and branches if it is set. The high bit of the value most recently affected will set the N flag. On 8-bit operations this is bit 7. On 16-bit operations (65816 only) this is bit 15.
This is normally used to determine if a two's-complement value is negative but can also be used in a loop to determine if zero has been passed when looping down through zero (the initial value must be positive)
BPL - Branch if Positive
BPL tests the Negative flag and branches if it is clear. The high bit of the value most recently affected will set the N flag. On 8-bit operations this is bit 7. On 16-bit operations (65816 only) this is bit 15.
This is normally used to determine if a two's-complement value is positive or if the high bit of the value is clear.
BVC - Branch if Overflow Clear
BVC tests the Overflow flag and branches if it is clear.
On the 6502 only 3 instructions alter the overflow flag: ADC, SBC & CLV.
On the 65C02 the BIT instruction also alters the overflow flag.
The PLP & RTI alter the flags as they restore all flags from the stack.
On the 65816 the SEP & REP instructions modify the v flag.
On some processors there's a Set Overflow hardware signal available, but on many systems there is no connection to that pin.
BVS - Branch if Overflow Set
BVS tests the Overflow flag and branches if it is set. It has the same limitations as the BVC instruction.
Flags Affected
| None. |
Instructions
| Syntax | Branch if | Opcode | Available on: | # of | # of | Addressing Mode | ||
|---|---|---|---|---|---|---|---|---|
| (hex) | 6502 | 65C02 | 65816 | bytes | cycles | |||
| BCC nearlabel | Carry clear | 90 | x | x | x | 2 | 21, 2 | Program Counter Relative |
| BCS nearlabel | Carry set | B0 | x | x | x | 2 | 21, 2 | Program Counter Relative |
| BEQ nearlabel | Equal, z=1 | F0 | x | x | x | 2 | 21, 2 | Program Counter Relative |
| BNE nearlabel | Not Equal, z=0 | D0 | x | x | x | 2 | 21, 2 | Program Counter Relative |
| BMI nearlabel | Minus, n=1 | 30 | x | x | x | 2 | 21, 2 | Program Counter Relative |
| BPL nearlabel | Positive, n=0 | 10 | x | x | x | 2 | 21, 2 | Program Counter Relative |
| BVC nearlabel | Overflow clear, v=0 | 50 | x | x | x | 2 | 21, 2 | Program Counter Relative |
| BVS nearlabel | Overflow set, v=1 | 70 | x | x | x | 2 | 21, 2 | Program Counter Relative |
Notes:
- Add 1 cycle if branch taken
- Add 1 more cycle if branch taken crosses page boundary on a 6502, 65C02 or a 65816 in 6502 emulation mode (e=1)