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PostPosted: Thu Dec 27, 2012 4:43 am 
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Joined: Sat Sep 15, 2012 9:49 am
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Location: Brisbane, Australia
Hi, everyone. I want to make sure that I understand this. Let me describe the situation as I think it is, and correct me if I'm wrong.

Within the 68000 CPU, there are two stack pointers, the user stack pointer and the supervisor stack pointer. When the CPU is in user mode, it uses the USP, which can be aliased as A7. Sometimes the CPU will enter supervisor mode. When it does so, it stops using USP and tucks the value away somewhere for safekeeping. While in supervisor mode, A7 references the SSP.

It is impossible to access the value of SSP while in user mode. While in supervisor mode, the USP can be accessed with a special opcode.

So at any given time, the 68000 is remembering the values of the two stack pointers, and A7 refers to one of them.

Am I right?


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PostPosted: Thu Dec 27, 2012 2:03 pm 
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Joined: Thu Dec 16, 2004 6:42 pm
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Correct :thumbleft:

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Prof. Kelly


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