Hand Assembling for the VAX..

[Keith Edwards]

>/***** ubc-bdcvax:comp.unix.wizards / e!kishore /  4:10 am  Mar 19, 1988*/
>The question I had is : given a BRW instructionthat looks like
>BRW 1040 (Branch Word) what does the bit pattern look
>like? (assuming the 1040 is in hex). Also BRW seems to allow a maximum 
>displacement of 16 bits. What does one use to make bigger jumps? 
>	Also if I obtain the address of a function bar using nlist, the actual
>function seems to starts a few bytes after the value returned by nlist. These
>few bytes are probably used to hold some status info. I've been trying to 
>figure out the exact size of this area. 

Here's the output for a brw #^x1040

                                                                                                                          page 1

                                    0000  00000000    1 	.entry _main,0
                                          00000002    2 
                              1040 8f 31  00000002    3 	brw #^x1040
                                          00000006    4 
                                          00000006    5 	.end _main
                                          00000006    6 

The Vax puts the more significant byte of each word at the higher addresses.

You can use the JMP instruction, which allows a 32-bit address target for
branches of longer that 16 bits.

The first 16-bit word of each function is a register mask which specifies
the registers which are to be saved on the stack when the procedure is
called.  The top 4 registers (PC, SP, AP, and FP) get saved automatically,
and you can't save r0 or r1 (which are used for function-returned values).
So to save all the registers you can save, use a register mask of ^x0ffc.

Hope this helps!

-keith