Bootrom functions about rp-hal HOT 11 CLOSED

Can you refer to the exact line where you see a 32-bit pointer read from the rom?

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We use rom_table_lookup which is supplied by the bootrom, so technically speaking we are not accessing the rom directly. If this function read 16-bit pointer value then we are fine :)

Hopefully it works like that. If not, well no pi pico has ever worked properly :O

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const FUNC_TABLE: *const *const u16 = 0x14 as _;

rom_table_lookup(FUNC_TABLE, some_tag);

fn rom_table_lookup<T>(table: *const *const u16, tag: RomFnTableCode) -> T {
    unsafe {
        let rom_table_lookup: RomTableLookupFn<T> = core::mem::transmute(ROM_TABLE_LOOKUP_PTR);
        rom_table_lookup(*table, u16::from_le_bytes(tag) as u32)
    }
}

The deref of the table pointer with *tablewill read four bytes (a *const _) from memory: 0x14 .. 0x17. You should only read 0x14 and 0x15 and add two zero bytes. You should probably set FUNC_TABLE to be a *const u16, then do a core::ptr::volatile_read(), then cast the u16 to a usize, then pass that to the rom_table_function.

In your ASM listing, I think LDR R0, [R0] will do a 32-bit read from the address 0x14. That LDR should be an LDRH.

But maybe I'm wrong, and this code works fine as-is? Maybe I've read the datasheet wrong?

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Note how the SDK code does a 16-bit read:

// Convert a 16 bit pointer stored at the given rom address into a 32 bit pointer
#define rom_hword_as_ptr(rom_address) (void *)(uintptr_t)(*(uint16_t *)rom_address)

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I think it's the same because jumping to 0x0014 is the same as jumping to 0x00000014 but essentially you are right

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And in thumb all 16 bit are extended to 32 bit register so yah...

I don't have the pico so I can't test it (:

Let me know if I am wrong

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Jumping to 0x0014 is the same as jumping to 0x00000014, but using the ROM table doesn't involve jumping to 0x0014 - it involves reading a two-byte value stored at 0x0014..0x0015.

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I didn't notice the double const*😳

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I am fixing it right now, why should I use core::ptr::volatile_read and not just regular deref?

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I would use it because it is more explicit that you are loading from a specific address in flash.

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