From db2e5de20bd3f0c42b115624d0919b397e8f6dad Mon Sep 17 00:00:00 2001 From: Erin Date: Sat, 22 Jul 2023 02:28:05 +0200 Subject: [PATCH] Moved lore --- hbasm/src/lib.rs | 49 ++++++++++++++++++++++++------------------------ 1 file changed, 25 insertions(+), 24 deletions(-) diff --git a/hbasm/src/lib.rs b/hbasm/src/lib.rs index 3235f5b..b779937 100644 --- a/hbasm/src/lib.rs +++ b/hbasm/src/lib.rs @@ -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]); } }