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