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The yellow paper says that the memory model for the EVM is a word-addressed byte array (YP 9.1), but when I run debug.traceTransaction in Geth, the memory looks more like an array of words, maybe a byte-addressed word array. Did this change between the YP and the actual implementation?

{
      depth: 1,
      gas: 3971707,
      gasCost: 3,
      memory: ["0000000000000000000000000000000000000000000000000000000000000000", "0000000000000000000000000000000000000000000000000000000000000000", "0000000000000000000000000000000000000000000000000000000000000060", "00000000000000000000000000000000000000000000000005d5349f8718c000"],
      op: "DUP1",
      pc: 2888,
      stack: ["00000000000000000000000000000000000000000000000000000000c6bd3052", "0000000000000000000000000000000000000000000000000000000000000080", "0000000000000000000000000000000000000000000000000000000000000060"],
      storage: {
        0000000000000000000000000000000000000000000000000000000000000006: "00000000000000000000000000000000000000000000000005d5349f8718c000"
      }
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2 Answers 2

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I view it as a byte-addressed byte-array.

It is certainly byte-addressed, in that consecutive integer addresses refer to consecutive bytes in memory.

It is true that you can only read a 32 byte word (mload reads 32 consecutive bytes), but reads are not word-aligned. mload(0) and mload(1) are both valid and have 31 bytes in common. so it is not really a word-array.

Writing is the same, except that you can also store a single byte with mstore8.

The only "word-array" aspect I can think of is that when memory is expanded, it is always rounded up to a multiple of 32 bytes. And this fact is only important for gas calculation.

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  • Thanks. After you explained it that way, I looked at the code and it seems to agree with you. The byte-addressed aspect is in the opMload function /core/vm/instructions.go and the byte-array aspect is in the Memory struct definition in /core/vm/memory.go
    – John Stanford
    Jan 7, 2018 at 18:51
  • Yes, I don't know why the YP says it is word-addressed. On the face of it this does not appear to be correct.
    – benjaminion
    Jan 7, 2018 at 18:53
  • 2
    By word-addressed it means that each address is a word. I.e. the address space is 2^256 since the address is one word (256 bits). It is a byte array in that each address points to a single byte, as opposed to storage, in which each address points to a full word.
    – Tjaden Hess
    Jan 8, 2018 at 18:27
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Memory is a word-addressed byte array. Storage is a word-addressed word array.

Geth simply chooses to display the memory in 32-byte segments, which is a bit of an odd choice, but when reading and writing to memory in the EVM each byte is independently accessable with mstore8

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  • mload8 is not a valid opcode. Reading memory always reads 32 bytes.
    – benjaminion
    Jan 7, 2018 at 18:18
  • Yup, that's right good catch
    – Tjaden Hess
    Jan 8, 2018 at 0:52

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