how to calculate opcode mask

[Shinpei Kato]

Hi, there.

I'm porting GAS to the new CPU that is similar to MIPS.
So I'm refering to binutils-2.14/opcodes/mips-opc.c to add several new
instructions.
However I have one thing I can't understand.
That is how I can caluculate a mask member of the type "mips_opcode".
For instance, please see followings.

const struct mips_opcode mips_builtin_opcodes[] = 
{
	:
	:
{"add", "d,v,t", 0x00000020, 0xfc0007ff, WR_d|RD_s|RD_t, I1},
	:
	:
}

This is the mips_builtin_opcodes definition in mips-opc.c.
0x0000000020 is a match member.
0x7c0007ff is a mask member.
I can understand a match member because it's just opcode but not a mask
member.
0xfc007ff means "d,v,t", doesn't it? But how can we calculate 0xfc0007ff
from "d,v,t"?

I hope your help.


Regards,
Shinpei Kato