CPSC 2310 - DAY 16
OCTOBER 10, 2016

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link register (lr) holds the return address

r0-r3 are parameter registers.
r4-r11 temporary register variables.  Must be preserved

REGISTER USAGE
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Temporary Variables
	.Functions should first try to use registers r0-r3 for any required
	temporary variables, since they do not have to be preserved
	.Registers r4-r11 are the second choice, but they must be saved
	at the function entry  and restored prior to function return
	.When more space for temporaries is required, allocate space on the
	stack

Reasons for using registers or stack space for temporary variables instead
of creating fixed locations with labels.

	1. Memory Conservation: the stack space will be released when the funct.
	returns so that it can be reused for other needs.

	2. Reentrancy: every entry of the function allocates a unique set of 
	memory locations for its temporary variables; therefore, if the funct.
	is interrupted, and the interrupting code calls (or reenters) the same
	function, it is ensured that each invocation will modify only its own
	set of temporary

A way to preserve and restore the registers is to push the registers onto the 
stack immediately upon entering the function, and to pop them back just before
the function returns

However, since the stack is stored in memory, and since memory cycles are the 
primary performance bottleneck, it is important to preserve a few registers
as possible.

r12 may be used by a linker as a scratch register between a routine and any
subroutine that calls it