# Bachelor thesis logbook

Camil Staps

### 2016-09-18
Made a Vim plugin for ABC code highlighting (camilstaps/vim-abc).

ARM/Thumb-2 mode switches are expensive, and since a Clean program continuously
switches between the RTS and the program itself, it is not viable to have the
RTS run in ARM mode and the code generator generate Thumb-2 code. Hence, I
started to rewrite the RTS for Thumb-2. Mostly minor, trivial changes:

 - Invisible registers for some instructions (auxiliary register needed)

Started to make changes to the code generator:

 - Added `.thumb`, `.syntax unified` to the top of each generated file
 - Added `.thumb_func` before each code label
 - Fix for the `str pc,[sp,#-4]!` and similar instructions, since `pc` cannot
   be the `Rd` operand in a `str` instruction: copy to scratch register first.

**Question**:
Is there an interface to get an auxiliary register in the code generator? For
example, to swap to registers a third is necessary (or the stack, but then the
PC and SP cannot be modified). In `w_as_jsr_instruction`, the PC is stored on
the stack, but PC cannot be `Rd` for `str`. Therefore we need an auxiliary
register (`8c91384` does this for some cases). *Answer on 09-19*.

### 2016-09-19
Talked to John:

 - (*Answer to question from 09-18*) Registers R14 (LR) and probably also R12
   are scratch registers and can be used at any time.
 - When the program counter is stored on the stack in ARM (`str pc,[sp,#-4]!`),
   the value pushed is actually the PC + 8, which gives space for a call after
   storing the PC. On Thumb-2 this is different, so this should be checked.