push and pop description in online manual

[Sohum Sohoni]

The manual currently lists that push translates to sw $rt, 0($sp); addiu $sp, $sp, -4, but in reality, what it does is addiu $sp, $sp, -4 sw $rt, 4($sp) as listed in the quick ref accompanying PLPTool. Same thing with pop, listed as addiu $sp, $sp, 4; lw $rt, 0($sp) in the manual, but implemented as lw $rt, 4($sp) addiu $sp, $sp, 4. This one's also correctly listed in the quick ref that you can pull up while using PLPTool.

Sohum

[Wira Mulia]

The quickref is correct. The reason for the order of instructions is to make these pseudo instructions interrupt-safe. We need to reserve the space on stack first in the case of pushing just in case an interrupt is triggered in between so that the space doesn't get clobbered if the ISR uses the stack. For popping, we get the data off the stack first, because otherwise an ISR will assume that the space is safe to use if we increment the stack pointer before getting the data off and the interrupt happens in between the two operations.

[Sohum Sohoni]

So should we update the manual then?

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[Wira Mulia]

I fixed it. I also changed the notes for pop operation (we are not popping $rt from the stack, we are popping data from the top of the stack into $rt).

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