# `btst`

[M68K Documentation](https://asm-editor.specy.app/documentation/m68k.md) › [Instructions](https://asm-editor.specy.app/documentation/m68k/instruction.md)

Web page: https://asm-editor.specy.app/documentation/m68k/instruction/btst

**Test a bit**

Category: Logic

Tests the bit in the second operand numbered by the first operand and sets Z to the inverse of its value.

Sizes .b .l

Used in: [Arithmetic, logic and bits](https://asm-editor.specy.app/learn/courses/m68k/think-in-assembly/arithmetic-logic-and-bits.md) (M68K assembly), [The screen, keyboard and mouse through traps](https://asm-editor.specy.app/learn/courses/m68k/talking-to-the-outside-world/screen-keyboard-and-mouse.md) (M68K assembly), [Bit tests, shifts, masks and counting set bits](https://asm-editor.specy.app/learn/courses/m68k/examples/bit-tricks.md) (M68K assembly), [Move a square with the keyboard](https://asm-editor.specy.app/learn/courses/m68k/examples/keyboard-control.md) (M68K assembly), [The snake game](https://asm-editor.specy.app/learn/courses/m68k/examples/snake.md) (M68K assembly), [Flappy ASM](https://asm-editor.specy.app/learn/courses/m68k/examples/flappy-asm.md) (M68K assembly), [Sampled sound](https://asm-editor.specy.app/learn/courses/m68k/examples/sampled-sound.md) (M68K assembly), [Arithmetic, logic and bits](https://asm-editor.specy.app/learn/courses/assembly-basics/think-in-assembly/arithmetic-logic-and-bits.md) (Assembly basics)

## Flags

| Flag | Effect |
| --- | --- |
| X | not changed |
| N | not changed |
| Z | depends on the result |
| V | not changed |
| C | not changed |

## Forms

- `btst <ea>, <ea>`
  - Source: Dn, #imm
  - Destination: Dn, (An), (An)+, -(An), d(An), d(An,Xn), abs, d(PC), d(PC,Xn), #imm (data)

## Operands

- [`<ea>`](https://asm-editor.specy.app/documentation/m68k/addressing-mode.md): An operand using one of the addressing modes listed for this form. The list determines whether it can be a register, memory or an immediate value. With `opt p=68020`, an allowed `d(An,Xn)` or `d(PC,Xn)` mode also permits the corresponding 68020 indexed and memory indirect forms. See [68020 addressing](https://asm-editor.specy.app/documentation/m68k/addressing-mode/68020.md).
- [`#imm`](https://asm-editor.specy.app/documentation/m68k/addressing-mode/immediate.md): An immediate value. As the first operand, it selects a bit: `0` to `31` for a data register, or `0` to `7` for memory.

## Example

```m68k
* --- Z flag reflects the ORIGINAL bit value ---
move.l #%0101, d0
btst #0, d0 ; bit 0 = 1, Z flag clear
move.l #%0101, d1
btst #1, d1 ; bit 1 = 0, Z flag set

* --- Test higher bits ---
move.l #$80000000, d2
btst #31, d2 ; bit 31 = 1, Z flag clear
```