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BIT: Difference between revisions
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'''BIT''' is a 65x instruction that performs a logical AND operation between the [[accumulator]] and memory without storing the conjunction. | '''BIT''' is a 65x instruction that performs a logical AND operation between the [[accumulator]] and memory without storing the conjunction. | ||
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]]. | 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. | ||
===== Cycle Penalties ===== | ===== Cycle Penalties ===== | ||
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=== See Also === | === See Also === | ||
* [[AND]] | * [[AND]] | ||
* [[BPL]] | |||
* [[BNE]] | |||
* [[BCC]] | |||
* [[BCS]] | |||
* [[BVC]] | |||
* [[BVS]] | |||
=== External Links === | === External Links === | ||
Revision as of 03:31, 25 November 2023
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Immediate | 89 | 2 bytes* | 2 cycles* | ||||
| Absolute | 2C | 3 bytes | 4 cycles* | ||||
| Direct Page | 24 | 2 bytes | 3 cycles* | ||||
| absolute indexed X | 3C | 3 bytes | 4 cycles* | ||||
| direct page indexed 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.
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.
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/n457
- Labiak page on BIT: https://archive.org/details/Programming_the_65816/page/n131
- MCS6500 Manual page on BIT: https://archive.org/details/mos_microcomputers_programming_manual/page/n62
- Carr page on BIT: https://archive.org/details/6502UsersManual/page/n262
- Leventhal page 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