Homework 3

Due Thursday, November 8

In this problem, you will write an assembly language file, SpecialStack.s, which manages a stack of your own, NOT one pointed to by ESP. The routines will be callable from C. Here are the details:

• SpecialStack.s will be self-contained; no stack manipulation code may be in the C callers.
• The stack grows away from 0. For example, a push increases the address of the top of the stack by 4.
• The address of the top of the stack is stored in a word with the label tos in the .data segment. (Or, if you prefer, a .comm segment.)
• You will allocate your first chunk of stack space by calling malloc(). (Calling brk() may be easier, but that requires more background.) If a push would result in stack overflow, then call malloc() to add another chunk of space. Set the chunk size in a .equ directive.
• The first two (i.e. two lowest-address) and last two (i.e. two highest-address) words in a stack chunk will be reserved for chunk linking:
  • The next-to-lowest-address and next-to-highest-address words will contain the value -1, signaling begin or end of chunk. (It will be assumed that no data item will ever be -1.
  • The lowest-address word will contain 0 if there is no preceding chunk; if there is such a chunk, this word will contain the address of the highest-address data word in that chunk.
  • The highest-address word will contain 0 if there is no following chunk; if there is such a chunk, this word will contain the address of the lowest-address data word in that chunk.

• Your code will check for stack underflow, meaning an error in which you attempt to perform a pop on an empty stack.
• Your SpecialStack.s file must contain the following functions, shown with their C signatures:
• void initstack(void): This sets up the first chunk of the stack, and initializes tos.
• void pushstack(int m): This pushes m onto the stack. Note that this may trigger the formation of a new chunk.
• int popstack(int *errcode): This pops the stack returning the popped value. Upon an underflow error, it will place an error code (1 for error, 0 if not) into errcode.
• void swapstack(void): This will swap the top two elements of the stack.
• void printstack(int n): This will print (in hex) the first n elements of the stack, starting from the top of the stack and working toward 0.

Other than as specified in popstack(), you need not write error-checking code.

Matt will be testing your code by calling it from C; make sure you check beforehand that that will work. Of course, set a small chunk size for easy testing.