Homework 7

22C:18, Summer 1997

Due Thursday, July 24

In lectures 29 and 30, the only macros actually defined for manipulating the stack were add, pushimm and select. In practice, you'd probably like others! Consider the following list of macros:

pushvar x: push the variable named x; assume that x is declared the way it is in the local and global macros from highlevel.h.
popvar x: push the variable named x; assume that x is declared the way it is in the local and global macros from highlevel.h.
pushaddr x: push the address of a variable named x; assume that x is declared the way it is in the local and global macros from highlevel.h.
get: push( M[pop()] ) -- follow a pointer on the stack top.
put: M[pop₂()] = pop₁() -- follow a pointer for store.
sub: push( pop₂() - pop₁() ) -- subtract.
neg: push( - pop() ) -- negate the stack top.
ls: push( pop₂() << pop₁() ) -- left shift.
ars: push( pop₂() >> pop₁() ) -- signed (arithmetic) right shift.
urs: push( pop₂() >> pop₁() ) -- unsigned right shift.
eq, gt, lt, ge, le, ne: push( pop₂() = pop₁() ) -- compare for equality.; push( pop₂() ? pop₁() ) -- the others are similar!

The following questions assume the use of these macros:

Convert each of the following to a string of calls to the above macros:

	a = b - (c << 2);

	b = a[ i ];  -- assume i is an array of integers

	a[ i ] = i + 5;  -- assume i is an array of integers

Write code for the ars macro.
Write code for the if, else and endif macros. These should allow you to rewrite
```
	if (i > 5) xxxx; else yyyy;
	
```
as the following:
```
	pushvar i
	pushvar 5
	gt
	if
	  xxxx
	else
	  yyyy
	endif
	
```
Your macros should follow the same scheme used for the loop macros in lecture 30.
Consider the select/case given in lecture 30. Here is a small bit of C code:
```
	select (i) {
	  case 0: xxxx; break;
	  case 1: yyyy; break;
	  otherwise zzzz; break;
	}
	
```
This can be written using our macros as:
```
	pushvar i
	select
	  case 0
	    xxxx
	    break
	  case 1
            yyyy
	    break
	  otherwise
	    zzzz
	    break
        endselect
	
```
Show the result of expanding these macros, so that the code is pure SMAL Hawk assembly language, with no macros other than the basic macros for Hawk machine instructions given in the hawk.macs file. You can get some help by actually typing this in and assembling it, but you'll find that the best way to solve this problem is to actually hand-expand the macros.
In the notes on the select/case macros, it noted that these macros differ from the C (and C++) select/case statements in one significant way. Can you find it? (and what is it?)