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Re: [MiNT] what's wrong with this code?

Patrice Mandin wrote:

Le Mon, 15 Aug 2005 20:52:21 +0100
<jan.thomas@bt.com> a écrit:



-----Original Message-----
From: mint-bounce@lists.fishpool.fi on behalf of Johan Klockars
Sent: Mon 15/08/2005 19:50
To: Mint list
Subject: Re: [MiNT] what's wrong with this code?

<SNIP>


There are physically separate registers for the various stack
pointers,  which just get "remapped" in hardware depending on the
bits in SR.


Question.

When you normally write the following...

       move.w D0,-(SP)   (or A7)

You write this instead...

       move.w D0,-(SSP)    or
move.w D0,-(USP) 

What happens when the CPU is in super or user mode?



ssp and usp are used through A7. Just look at generated assembler code.
The only difference is the 'move from usp' privileged instruction, used
for example in most system traps to read parameters on user stack, from supervisor mode:

my_xbios_trap:
	lea	sp@(8),a0	; start of stacked parameters if trap #14 from supervisor mode
	btst	#5,sp@			; check S bit in stacked SR
	bnes	supervisor_mode
	movl	usp,a0			; read user stack pointer if trap #14 from user mode
supervisor_mode:
	movew	a0@,d0		; read xbios function number

	[etc]

	rte



Another unspecific example of usp/ssp mechanism:

trap enters SV mode --> A7 represents ssp, SR saved on the stack,
real SR gets SV bit.
If trap was issued in user mode and you set SV bit in SR on stack
rte will not enter user mode and A7 will keep representing the ssp
The usp itself will be unchanged.

Because you are still in SV, you can clear SV bit in real SR and lo
and behold, you are in user mode and A7 represents the usp again.

andi #notSV,SR

Correct?

Things can go weird if you move something to A7 somewhere.
For instance if you move usp,A7 when in SV mode. ssp:=usp. :-)

A short journey into the unknown.
To go boldly where no one has returned before.

--
Groeten; Regards.
Henk Robbers.    mailto:h.robbers@chello.nl
                   http://members.ams.chello.nl/h.robbers/Home.html
Interactive disassembler:     TT-Digger;  http://digger.atari.org
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