Gecko Guide

(An intro to Gecko and how it’s used in this project. Optional reading is in italics.)

Gecko is an assembly-like language for working directly with memory and/or triggering custom ASM. The codes are interpreted by a “codehandler” (in an emulator or loader) that runs them at least once per frame by injecting them into standard system calls – usually OSSleepThread on Nintendont, also often Vertical Blanking Interval (between frame draws), etc. This means they carry no state between runs, and will repeat their actions each time. The exception to this is the CC code and Switch idiom.

Codetypes

The syntax is pairs of 4B words, usually written in hex. The codetype is the first byte of the (8B) pair. The following are used in this project. Docs here.

Read/Write

00–05; 10–15 – Write literal
Write literal (= constant) directly to memory.

80; 82; 84 – GR read/write
Set Gecko register (GR) to literal, or copy from/to it to/from memory. These are a set of 16 4B memory locations provided by Gecko.

42; 4A – Set default address
Set base/pointer (default) addresses (BA/PA) to literal. Memory addresses in other codes are usually interpreted as offsets relative to these, so they can fit into the 8B codes. See Address and Pointer idioms.

If

20–3F – If literal
Compare memory to literal (=, ≠, <, >). There is no direct way to compare GRs to literals.

CC – On/Off
Detect change in if result between runs. Persists state, so use this for codes that toggle things – see Switch idiom.

DE – Assert pointer
Checks that a value in the pointer address (PA) is valid. Use for safety before dereferencing; see Pointer idiom.

E0 – Infinite end-if
Ends all conditional statements.

E2 – Finite end-if
Ends specified number of conditional statements. A single end-if can be applied directly from an if literal by adding 1 to the mem address (note that 1B if literal doesn’t exist).

ASM

C0 – Execute ASM
Run ASM instructions within the Gecko code. This and C2 need a separate guide!

C2 – Inject ASM
Inject ASM into an existing function.

Notes

Data sizes

Most read/write commands require choosing data size, from 1B, 2B or 4B. The intended size of a variable is usually visible in Ghidra.

If statements allow comparison of 2B or 4B variables.

Branching

The implementation of if statements (20–3F; DE) is unlike structured languages. Imagine a signal (“code execution”) passing through the code, that gets disabled (possibly repeatedly) if the conditional evals to false. End-ifs can be applied to clear specific quantities of disablings (E2) or all of them (E0). The CC code is different – see Switch idiom.

Idioms

Address

Base address is usually set to 80000000, which makes all of GC memory accessible by adding the codetype (writing e.g. 04 as 04000000) to the address (resulting in a range of e.g. 04000000 to 05200000). A convenient way to access Wii memory is to find (or set) a permanent variable to 90000000, then load its address to the pointer address. Then BA covers GC and PA covers Wii.

Changing BA/PA to non-x0000000 values to use offsets is a faff imo bc of the lost direct comparison with Dolphin Memory Engine among other things.

Pointer

Otoh, a situation where PA must be used is to access heap data that is at risk of moving. Find a static pointer to the containing object, then:

  1. load it to PA (with 4A code);
  2. validate that it’s a pointer (with DE code);
  3. dereference it with offset (in any other code).

Flag

The executable part of memory (typically at addresses ~80004000 to ~80400000 or ~80500000) is guaranteed to not change and has lots of spaces left by alignment of functions. I use this to store flags and Gecko registers to back-up data, because GRs can’t be compared to literals.

Copy

Directly copying from memory to memory is not possible (copying between memory and GRs is). To alleviate that, copy to a “volatile” GR, then copy from it to elsewhere. Convention: volatile GRs will be the ones counting backwards from index E. Don’t use F bc it’s a special char in various GR codetypes signalling not to use a GR at all!

For more complicated or bulk copying, find the memcpy function in the game and call it with C0 ASM execution.

Switch

The key is that a CC code executes regardless of all if conditional disabling of execution. It toggles off/on if and only if it’s hit by an enabled signal and was on the last run hit by a disabled signal. Key example: atomic button press.

The switch idiom looks like this:

[condition]         # 1. If (I)
[true payload (T)]  # 2
cc000000 00000000   # 3. Switch (S)
[same condition]    # 4. Same (I)
[false payload (F)] # 5
e0000000 00000000   # 6. ∞ end-if

Example of condition (I): if d-pad right is pressed in Skyward Sword:

2859CF8C 00000002

I is true/false (+/-) and S is on/off (+/-).
Statically, this happens:

I+ S+: runs T, then F
I- S+: doesn't run
I+ S-: runs T
I- S-: doesn't run

Dynamically, we have this loop:
(initial state is I- S+)
( denotes changes, ~ no change, and : causality)

# I→+ S+: S→- : forces T
# I→- S-: S~  : persists above
# I→+ S-: S→+ : forces T, then F
# I→- S+: S~  : persists above

(I) is typically a button-press conditional, which is great because this means T or F will only execute while trigger buttons are pressed (and their results will remain when unpressed), so have no potential lag impact on gameplay. Likewise, the same condition toggles between applying T and F, so one button can toggle something.

The key downside is that F cannot be run separately, only T then F. Fine if F overwrites the same values as T, but otherwise note side-effects and things set in T that should be reverted by F.

Note: (I) can be replaced with multiple lines, interpreted as logical conjunction, w/o any change in the idiom.

Warning: if using an end-if in (I) (by incrementing the mem address), the second instance of (I) must not end-if (put it back to the original mem address). The F payload is conditional on the switch; end-if would make it accessible from outside.