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Re: Some more patches for GCC on NetBSD/VAX coming soon...



On 2016-04-01 22:28, Mouse wrote:
e1:	.byte	0xd0
e2:	.byte	0x53
	.byte	0x50

If entered at e1, this is "movl r3,r0"; if entered at e2, it's
"tstf r0".

Yes. But the problem is that the opcode at e2 will be interpreted as
an operand for e1.

That may or may not be a problem; if it is a problem then you have't
been sufficiently clever. :-)

And as an operand it might imply that it needs up to 8 bytes more or
so...

Yes.  It requires care and cleverness to do anything useful with this.

Which will totally break the concept.

Hardly "totally break"; it just means it's not a no-brainer to use.

It _does_, though, mean that it will be substantially harder to use in
an automated way; except for human-created canned sequences it is
unlikely to be of any use in gcc.

All true. But half the point in the original example code was that you can have any instruction there, which then can be skipped by this clever "prefix". This general approach will not work on a VAX. Yes, for some specific opcodes, you will be able to do it, but it's not a generic solution here.

	Johnny



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