Question

Define: EA = (X)+ is the effective address equal to the contents of location X, with X incremented by one word length after the effective address is calculated; is the effective address equal to the contents of location X, with X decremented by one word length before the effective address is calculated; is the effective address equal to the contents of location X, with X decremented by one word length after the effective address is calculated. Consider the following instructions, each in the format (Operation Source Operand, Destination Operand), with the result of the operation placed in the destination operand.
a. OP X, (X)
b. OP (X), (X)+
c. OP (X), (X)
d. OP (X), (X)
e. OP (X), (X)+
f. OP (X), (X)+
g. OP (X), (X)
Using X as the stack pointer, which of these instructions can pop the top two elements from the stack, perform the designated operation (e.g., ADD source to destination and store in destination), and push the result back on the stack? For each such instruction, does the stack grow toward memory location 0 or in the opposite direction?

 

Answer 1

a.         No, because the source operand is the contents of X, rather than the top of the stack, which is in the location pointed to by X.

 

b.         No, because address of the top of the stack is not changed until after the fetching of the destination operand.

 

c.         Yes. The stack grows away from memory location 0.

 

d.         No, because the second element of the stack is fetched twice.

 

e.         No, because the second element of the stack is fetched twice.

 

f. No, because the stack pointer is incremented twice, so that the result is thrown away.

 

 

g.         Yes. The stack grows toward memory location 0.