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# 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.
To solve the latter problem, we need to add an offset to the stored PC. Old ARM
code was:
```armasm
str pc,[sp,#-4]! @ Actually pushed PC+8, to after the branch.
```
First solution, since `PC` cannot be `Rd` for `str`:
```armasm
mov r12,pc @ Stores PC+4 (see A5.1.2 of the ARMv7-A manual) (this is a 16-bit instruction)
str r12,[sp,#-4]! @ Pushes stored value (address of THIS register)
```
New solution, with a proper offset:
```armasm
add r12,pc,#9 @ Stores PC+12+1 (this is a 32-bit instruction)
@ +1; bit 0 is the execution state and should be Thumb-2
str r12,[sp,#-4]! @ This is a 32-bit instruction, as is the branch below
```
After this, this program crashes only when it wants to print time information
(specifically, `time_string_4`, `thumb2startup.s:929`):
```clean
Start = 10
```
The last crash has to do with alignment in `exit_clean` (`thumb2startup.s`).
This indicates that `add r12,pc,#9` is not right in some cases. The second
bullet point in A5.1.2 applies:
> Read the word-aligned PC value, that is, the address of the current
> instruction + 4, with bits [1:0] forced to zero.
Hence, in some cases (when the address of the `add` instruction is 2 mod 4), we
should add 11 instead of 9. A quick fix is to use `.align`, which inserts a
`nop` if needed. It would be more convenient if there would be preprocessor
directives like `.align` where you could yourself decide what is added
depending on the current alignment (see
http://stackoverflow.com/q/39582517/1544337 for this).
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