Help needed for porting opcodes for CISC architecture

[Usha Gupta <usha.nitt@gmail.com>]

Hi All ,

I am  new to CGEN and want to port binutils for a CISC  architecture
wherein the instruction length can vary from 1 byte to a maximum of
5-bytes.

I am having trouble porting opcodes using CGEN.

I have defined various hardware elements and instruction fields.

I am not sure what should be the values of

(default-insn-bitsize 8)
(base-insn-bitsize 16)
(default-insn-word-bitsize 16 )
(word-bitsize 16 )

As mentioned in the CGEN manual, I specified default-insn bitsize  as
8 (smallest instruction length) and rest of the values as 16 ( word
length is 2 bytes).

This works for 16-bit instructions where both the operands are
explicit in the instruction encoding.

Another important thing to mention here is that the machine does not
have a fixed length opcode.

Here are some of the instruction formats :

1-byte instruction:
1)      IIIIIrrr - 5 bits opcode , 3 bits for register operand (one of
the operand is fixed register, implied from the opcode)
2)      IIIddddd - 3 bits opcode, 5 bit displacement


2-byte instruction:
1)      IIIIIsss IIIIIttt - 5-bit opcode, 3-bit register  operand
(source), 5-bit opcode, 3-bit register operand (destination)

How do I define  instructions of varying length?

Any help is appreciated.

Thanks and Regards,
Usha Gupta