Re: GAS ARM config/tc-arm.c

[Nick Clifton <nickc at redhat dot com>]

Hi Michael,

>  X(ldr,   6800, f8500000),                     \
> and
> tCE(ldr,       4100000, ldr,      2, (RR, ADDR),    ldst, t_ldst),
>
> Can someone provide me with a description of how the numbers 6800, 
> f8500000, and 4100000 are derived?

OK, so the "6800" is the 16-bit LDR(1) instruction.  If you look in the 
ARM Architecture Reference Manual you will see that the LDR(1) 
instruction must have the top five bits set to 0 1 1 0 1, or in a hex 
encoding using 16 bits: 0x6800.  Hence the "6800".

Now I do not have a 32-bit Thumb ISA specification to hand, but I expect 
that the principle is exactly the same.  ie "f8500000" is the 32-bit 
encoding of the top N-bits of the LDR(1) instruction.

Similarly the 32-bit ARM LDR instruction must have bits 20 and 26 set 
which, in hex is 0x04100000.  Hence the "4100000" pattern in the other 
macro you mention.

Cheers
  Nick