Computers as Components, 2nd Edition

234 CHAPTER 5 Program Design and Analysis

to an element in the frame by addressing relative to sp. When a new procedure is

called, the sp and fp are modified to push another frame onto the stack.

The ARM Procedure Call Standard (APCS) is a good illustration of a typi-

cal procedure linkage mechanism. Although the stack frames are in main memory,

understanding how registers are used is key to understanding the mechanism, as

explained below.

■ r0 ⫺ r3 are used to pass parameters into the procedure. r0 is also used to hold

the return value. If more than four parameters are required, the y are put on

the stack frame.

■ r4 ⫺ r7 hold register variables.

■ r