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BIT
From SnesLab
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Immediate | 89 | 2/3 bytes | 2 cycles* | ||||
| Absolute | 2C | 3 bytes | 4 cycles* | ||||
| Direct Page | 24 | 2 bytes | 3 cycles* | ||||
| Absolute Indexed by X | 3C | 3 bytes | 4 cycles* | ||||
| Direct Page Indexed by X | 34 | 2 bytes | 4 cycles* | ||||
| Flags Affected | ||||||||
|---|---|---|---|---|---|---|---|---|
| Addressing Mode | N | V | M | X | D | I | Z | C |
| Immediate | . | . | . | . | . | . | . | |
| other | . | . | . | . | . | |||
BIT is a 65x instruction that performs a logical AND operation between the accumulator and memory without storing the conjunction. If the conjunction is zero, the zero flag is set, otherwise it is cleared.
Except in immediate addressing, the most significant bit of the data located at the effective address is moved into the negative flag, and the second most significant bit of that data is moved into the overflow flag. BIT is often used right before a conditional branch instruction like BVC or BVS.
Syntax
BIT #const BIT addr BIT dp BIT addr, X BIT dp, X
Cycle Penalties
- BIT takes an extra cycle when the accumulator is 16 bits wide, in all addressing modes
- In direct page addressing modes, BIT takes an extra cycle when the low byte of the direct page register is nonzero
- In Absolute Indexed, X Addressing, BIT takes an extra cycle when adding the index crosses a page boundary
See Also
External Links
- Eyes & Lichty page 431, on BIT: https://archive.org/details/0893037893ProgrammingThe65816/page/431
- Labiak page 121 on BIT: https://archive.org/details/Programming_the_65816/page/n131
- MCS6500 Manual page 47 on BIT: https://archive.org/details/mos_microcomputers_programming_manual/page/n62
- Carr page 249 on BIT: https://archive.org/details/6502UsersManual/page/n262
- Leventhal page 3-45 on BIT: https://archive.org/details/6502-assembly-language-programming/page/n94
- snes9x implementation of BIT: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L265
- undisbeliever on BIT: https://undisbeliever.net/snesdev/65816-opcodes.html#bit-test-memory-bits-against-accumulator
- Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#BIT
- Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.2.2