CODEWITHSUNDEEP
webassembly
conr2d
conr2d

Reputation: 239

Why is `0x83 0x00` well-formed encoding of 3 as a u8 in Wasm?

enter image description here

(source: http://webassembly.github.io/spec/core/binary/values.html#integers)

According to my understanding of specification, value 3 should be encoded to 0x03 for u8. (It satisfies the condition of "if n < 27 ∧ n < 2N")

By the way, the next paragraph says like this:

The side conditions N>7 in the productions for non-terminal bytes of the u and s encodings restrict the encoding’s length. However, “trailing zeros” are still allowed within these bounds. For example, 0x03 and 0x83 0x00 are both well-formed encodings for the value 3 as a u8.

I get 0x83 0x00 by the second equation, but it's for the case of "if n ≥ 27 ∧ N < 7". For value 3, it doesn't satisfy the condition of "n ≥ 27" How can 0x83 0x00 be a well-formed encodings in this case?

Upvotes: 1

Views: 178

Answers (1)

Andreas Rossberg
Andreas Rossberg

Reputation: 36118

You may be interpreting the specification in the wrong direction. It specifies decoding, not encoding. So in the example, n is not the value 3, but the byte value 0x83. 3 should equal the right-hand side formula "2^7 * m + (n - 2^7)". This is the case because m (the value of the second byte) will be 0.

Upvotes: 2

Related Questions