mips instruction encoding

Instructions are divided into three types: R, I and J. Every instruction starts with a 6-bit opcode. In addition to the opcode,
R-type instructions specify three registers, a shift amount field, and a function field;
I-type instructions specify two registers and a 16-bit immediate value;
J-type instructions follow the opcode with a 26-bit jump target.

The following are the three formats used for the core instruction set:
Type format (bits)

      31-26 | 25-21 |20-16 |15-11 | 10-6 | 5-0 |
R: opcode (6) | rs (5) |rt (5) |rd (5) | shamt (5) | funct (6) |
I: opcode (6) | rs (5) |rt (5) |immediate (16) |
J: opcode (6) | address (26)                         |


UDI instructions are added to MIPS24Kc core.