notes

ARM Notes:
- Exception levels provide a logical separation of software execution privilege that applies across all
operating states of the ARMv8-A architecture
- The ARMv8-A architecture also defines two Execution States, AArch64 and AArch32. AArch64 state
is unique to ARMv8-A, and uses 64-bit general-purpose registers, while AArch32 state provides
backwards compatibility with ARMv7-A using 32-bit general-purpose registers
- _ELx refers to register from target exception level x, _ELs refers to register in the source exception level s
- SPSR (Saved Program Status Register)
- When the PE takes an exception, the PE state is saved from PSTATE in the SPSR at the Exception level the
exception is taken to. For example, if the PE takes an exception to EL1, the PE state is saved in SPSR_EL1.
- Exceptions cannot be taken to EL0 
- exceptions never cause execeution to move to a lower Exception level
- an exception return is always to the same Exception level or a lower exception level
    -   an exception return requires the simultaneous restoration of teh PC and PSTATE to the calues that are consistent witht he desired 
        state of execution on returning from the exception
- synchronous exception is caused by excecution or attempted execution of the instruction stream, and where the 
return address provides details of the instruction that caused it
- The location in memory where the handler is stored is called the exception vector
-  In the ARM architecture, exception vectors are stored in a table, called the exception vector table.
- each exception level has its own vector table, EL1, EL2, EL3
- the table contains instructions to execute rather than a set of addresses

?- why does the kernel seem to boot in EL2 instead of EL3? When I don't call switch to EL2, it still shows my current EL as 2?


- why is the frame pointer being used in the HANDLER function to store the exceptino kind and source?
- which esr to use in context_save?
- why can you not cast from &u64 to u64 but you can cast from *const u64 to u64 (init.rs)
- how would you call an assembly function from HLL code?

Phase 2:
- Unique in Rust?
- What would happen if you didn't implement Drop for Stack? 
    Why dont we have to implement this for everytype we create?


Phase 3:
- Why does the L2Table have 8192 entries even though the description says it will only contain 
2 entries?
- Why are virtual addresses 64 it but then when you print a pointer rn it is 32 bits?
- What's the difference between VirtualAddress and L2/L3 Raw Entry? (See locate() function)
- WHy would you ever use set_masked()?
- why is T0SZ=34 (4GB) but T1SZ=34 (1GB) 
- How do we ensure that all user processes only produce virtual addresses above USER_IMG_BASE
- How much virtual address space does the kernel have? (paragraph right above Page Table Entry header)
 
 Phase 4:
 - permissions of UserPageTables like th stack