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need help with stack pointers

now before i begin, i am newbie to programming. i've never taken any typ of programming and i am in assembler language class. we're studyin motorola 6800 and ran into a question

the profesor tells us that when using such commands as bsr,rts, and movem the stack pointer value will change. he also tells that we have t subtract the offset value from the current stack pointer value to get th current value of the stack pointer.

is that true or is he mistaken? because i would think that you would hav to add the offset value.

for example in command movem d0-d5/d7/a0-a3, since there are 44 byte involved, the new stack pointer value would be 102C, i would think

44, converted into hex is 2C and 1000+2C= 102C
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350ZBOI
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According to these 68HC05 data sheets, the stack starts at a high address and grows downward. You can see evidence of this when the BSR instruction subtracts from the stack pointer as it pushes a return address, and the RTS instruction adds to the stack pointer as it pops that address back into PC.

So if you're going to establish a frame on "top" of such a stack, you have to avoid the higher addresses which are already in use, and put your frame in locations below the current SP value. It would be possible to locate a "frame base" one frame size below the SP value, get that in a register, and index up from there, but that wastes a register for no benefit. Simplest to index negative from the SP.

Mel.

Reply to
Mel Wilson

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