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Moved lore

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Erin 2023-07-22 02:28:05 +02:00
parent 3892a719eb
commit 8d5d22eae1
1 changed files with 25 additions and 24 deletions
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49
hbasm/src/lib.rs
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@ -47,31 +47,32 @@ impl Assembler {
}
/// Append 12 zeroes (UN) at the end
///
/// # HBVM lore
///
/// In reference HBVM implementation checks are done in
/// a separate phase before execution.
///
/// This way execution will be much faster as they have to
/// be done only once.
///
/// There was an issue. You cannot statically check register values and
/// `JAL` instruction could hop at the end of program to some byte, which
/// will be interpreted as opcode and VM in attempt to decode the instruction
/// performed out-of-bounds read which leads to undefined behaviour.
///
/// Several options were considered to overcome this, but inserting some data at
/// program's end which when executed would lead to undesired behaviour, though
/// not undefined behaviour.
///
/// Newly created `UN` (as UNreachable) was chosen as
/// - It was a good idea to add some equivalent to `ud2` anyways
/// - It was chosen to be zero
/// - What if you somehow reached that code, it will appropriately bail :)
/// - (yes, originally `NOP` was considered)
///
/// Why 12 bytes? That's the size of largest instruction parameter part.
pub fn finalise(&mut self) {
// HBVM lore:
//
// In reference HBVM implementation checks are done in
// a separate phase before execution.
//
// This way execution will be much faster as they have to
// be done only once.
//
// There was an issue. You cannot statically check register values and
// `JAL` instruction could hop at the end of program to some byte, which
// will be interpreted as opcode and VM in attempt to decode the instruction
// performed out-of-bounds read which leads to undefined behaviour.
//
// Several options were considered to overcome this, but inserting some data at
// program's end which when executed would lead to undesired behaviour, though
// not undefined behaviour.
//
// Newly created `UN` (as UNreachable) was chosen as
// - It was a good idea to add some equivalent to `ud2` anyways
// - It was chosen to be zero
// - What if you somehow reached that code, it will appropriately bail :)
// - (yes, originally NOP was considered)
//
// Why 12 bytes? That's the size of largest instruction parameter part.
self.buf.extend([0; 12]);
}
}