index.md

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title: Count trailing zeros
slug: WebAssembly/Reference/Numeric/Count_trailing_zeros

---

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The ctz instructions, short for count trailing zeros, are used to count the amount of zeros at the start of the numbers binary representation.

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Syntax

;; load a number onto the stack
i32.const 8388608 ;; 00000000_10000000_00000000_00000000

;; count trailing zeros
i32.ctz

;; the top item on the stack will now be 23

Instruction	Binary opcode
i32.ctz	0x68
i64.ctz	0x7a