Bytecode version differences

v15 vs v16

TODO: test if these are actually bytecode related, or just a newer version of GMS runtime (like with the padding or GMS2 stuff)

v15	v16
No LANG chunk	Contains a new LANG chunk
No ROOM.GameObject.PreCreate	Contains a new ID reference to a Pre-Create code entry

These two changes is literally all I could see. Let me know if you can find anything else.

v14 vs v15

To begin, here are the major file format updates between the two.

v14	v15
No code local information	The end of the FUNC chunk has code locals
VARI chunk does not have initial 3 integers, variables do not have InstanceType or ID	VARI chunk has initial 3 integers (InstanceVarCount, InstanceVarCountAgain, MaxLocalVarCount), variables have InstanceType and ID
FUNC chunk is only an array with NO length, containing function information	FUNC chunk has an array WITH length, containing function information, followed by an array (with length) containing code local information
Each code entry has a string pointer to its name, followed by int32 length, immediately followed by the bytecode (using length)	Each code entry has a string pointer to its name, followed by int32 length, followed by int32 code local count, followed by int32 bytecode relative address, followed by int32 offset within the bytecode

Additionally, from bytecode 14 to 15, the entire instruction set was changed. Here's a list of the basic operation opcode changes, with their patterns

Name	v14 Opcode	v15 Opcode
(Pattern for operations)	v15 opcode - 0x04	v14 opcode + 0x04
Conv	0x03	0x07
Mul	0x04	0x08
Div	0x05	0x09
Rem	0x06	0x0A
Mod	0x07	0x0B
Add	0x08	0x0C
Sub	0x09	0x0D
And	0x0A	0x0E
Or	0x0B	0x0F
Xor	0x0C	0x10
Neg	0x0D	0x11
Not	0x0E	0x12
Shl (shift left)	0x0F	0x13
Shr (shift right)	0x10	0x14
Pop	0x41	0x45
Dup	0x82	0x86
(Pattern for most others)	v15 opcode + 0x01	v14 opcode - 0x01
Ret	0x9D	0x9C
Exit	0x9E	0x9D
Popz	0x9F	0x9E
B	0xB7	0xB6
Bt	0xB8	0xB7
Bf	0xB9	0xB8
PushEnv	0xBB	0xBA
PopEnv	0xBC	0xBB
Call	0xDA	0xD9
(Exceptions)
Cmp (not in v14 as one)	0x11 to 0x16	0x15
Cmp (<)	0x11	N/A
Cmp (<=)	0x12	N/A
Cmp (==)	0x13	N/A
Cmp (!=)	0x14	N/A
Cmp (>=)	0x15	N/A
Cmp (>)	0x16	N/A
Push	0xC0	0xC0
PushLoc	N/A	0xC1
PushGlb	N/A	0xC2
PushVar (builtin)	N/A	0xC3
PushI (int16)	N/A	0x84
Break (unchanged)	0xFF	0xFF

In this list are some major exceptions - the Cmp instruction(s) and Push instruction(s).

For the Cmp instructions in bytecode 14, the ComparisonType parameter is unused (zero), and the opcode takes its place. Conveniently, they are in the same order as ComparisonType.

For the Push instruction in bytecode 14, it obviously only has one opcode instead of bytecode 15's five. This means that it depends on the instruction's data type heavily. To put it simply, if the data type is Variable, then the first two bytes of the instruction have the variable scope as an int16, be it -5 (global, PushGlb), -6 (builtin, PushVar), or -7 (local, PushLoc). Other types such as -1 (self) also exist, but is unnecessary because it already assumes them. Also, if the data type is Int16, then the instruction is the equivalent of bytecode 15's PushI. As a side note, this means that instructions like push.e and pushi.e are now equivalent.

One other final note is that exiting a with statement early produces a PopEnv with a magic number (in both versions), and in the case of bytecode 14, it seemingly produces 00 00 F0 BC.

v13 vs v14

TODO: test if these are actually bytecode related, or just a newer version of GMS runtime (like with the padding or GMS2 stuff)

v13	v14
No AGRP chunk	Contains a new AGRP chunk
No debugger port	Has a debugger port in GEN8
Different font padding	Consistent font padding

It's also worth noting that some version within bytecode 13 changed the OPTN chunk drastically. Previously there were numerous 32-bit booleans for options, and these became a single 64-bit bitflag.