@@ -102,6 +102,10 @@ Consequently, the return value of read/write syscalls is at most 4, indicating t
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@@ -102,6 +102,10 @@ Consequently, the return value of read/write syscalls is at most 4, indicating t
Writing to stderr/stdout standard stream always succeeds with the write count input returned.
Writing to stderr/stdout standard stream always succeeds with the write count input returned.
Reading from stdin has no effect other than to return zero and errno set to 0.
Reading from stdin has no effect other than to return zero and errno set to 0.
### Hint Communication
Hint requests and responses have no effect on the VM state other than setting the `$v0` return register to the requested read/write count. VM implementations may utilize hints to setup subsequent pre-image requests.
### Pre-image Communication
### Pre-image Communication
The `preimageKey` and `preimageOffset` state are updated via read/write syscalls to the pre-image read and write file descriptors (see [I/O](#io)).
The `preimageKey` and `preimageOffset` state are updated via read/write syscalls to the pre-image read and write file descriptors (see [I/O](#io)).
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@@ -113,6 +117,13 @@ When handling pre-image reads, the `preimageKey` is used to lookup the pre-image
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@@ -113,6 +117,13 @@ When handling pre-image reads, the `preimageKey` is used to lookup the pre-image
A max 4-byte chunk of the pre-image at the `preimageOffset` is read to the specified address.
A max 4-byte chunk of the pre-image at the `preimageOffset` is read to the specified address.
Each read operation increases the `preimageOffset` by the number of bytes requested (truncated to 4 bytes and subject to alignment contraints).
Each read operation increases the `preimageOffset` by the number of bytes requested (truncated to 4 bytes and subject to alignment contraints).
#### Pre-image I/O Alignment
As mentioned earlier in [memory](#memory), all memory operations are 4-byte aligned. Since pre-image I/O occurs on memory, all pre-image I/O operations must strictly adhere to alignment boundaries. This means the start and end of a read/write operation must fall within the same alignment boundary.
If an operation were to violate this, the input `count` of the read/write syscall must be truncated such that the effective address of the last byte read/writtten matches the input effective address.
The VM must read/write the maximum amount of bytes possible without crossing the input adress alignment boundary. For example, the effect of a write request for a 3-byte aligned buffer must be exactly 3 bytes.
If the buffer is misaligned, then the VM may write less than 3 bytes depending on the size of the misalignment.
## Exceptions
## Exceptions
The FPVM may raise an exception rather than output a post-state to signal an invalid state transition. Nominally, the FPVM must raise an exception in at least the following cases:
The FPVM may raise an exception rather than output a post-state to signal an invalid state transition. Nominally, the FPVM must raise an exception in at least the following cases:
