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LSR: Difference between revisions
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==== Cycle Penalties ==== | |||
* Except in [[accumulator addressing]], LSR takes an extra two cycles when the accumulator is 16 bits wide | * Except in [[accumulator addressing]], LSR takes an extra two cycles when the accumulator is 16 bits wide | ||
* In [[direct page addressing]] modes, LSR takes another extra cycle if the low byte of the [[direct page register]] is nonzero. | * In [[direct page addressing]] modes, LSR takes another extra cycle if the low byte of the [[direct page register]] is nonzero. | ||
Latest revision as of 17:45, 23 August 2024
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Accumulator | 4A | 1 byte | 2 cycles | ||||
| Absolute | 4E | 3 bytes | 6 cycles* | ||||
| Direct Page | 46 | 2 bytes | 5 cycles* | ||||
| Absolute Indexed by X | 5E | 3 bytes | 7 cycles* | ||||
| Direct Page Indexed by X | 56 | 2 bytes | 6 cycles* | ||||
| Flags Affected | ||||||||
|---|---|---|---|---|---|---|---|---|
| N | V | M | X | D | I | Z | C | |
| 0 | . | . | . | . | . | Z | C | |
LSR (Logical Shift Right) is a 65x instruction that shifts every bit of a value one bit to the right (division by two). The most significant bit and negative flag are cleared. The least significant bit is shifted into the carry flag. The previous value of the carry flag is lost (unlike with ROR).
The size of the accumulator determines how many bits are shifted (8 or 16) not including the clearing zero and carry flag.
Syntax
LSR LSR A LSR addr LSR dp LSR addr, X LSR dp, X
Cycle Penalties
- Except in accumulator addressing, LSR takes an extra two cycles when the accumulator is 16 bits wide
- In direct page addressing modes, LSR takes another extra cycle if the low byte of the direct page register is nonzero.
See Also
External Links
- Eyes & Lichty, page 465 on LSR
- lbid page 191, before & after diagram of LSR
- Labiak, page 152 on LSR
- 10.1 on MCS6500 Manual, page 148 on LSR
- Carr, page 265 on LSR
- Leventhal, page 3-76 on LSR
- snes9x implementation of LSR: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L862
- undisbeliever on LSR: https://undisbeliever.net/snesdev/65816-opcodes.html#lsr-logical-shift-right
- Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#LSR
- Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.3