Page 351 - Programming Microcontrollers in C

336 Chapter 6 Large Microcontrollers

and stack; program, multiple 65 K banks for code and one 65 K bank
for data and stack; data, one 65 K bank for code and multiple 65K
banks for data and stack; and finally far, multiple 65 K banks for code,
data, and stack. With each of these memory models, it is necessary to
keep track and often change the contents of the extension registers. As
a result, with these models, it is necessary to alter slightly the stacking
sequence. Such a sequence is shown in Figure 6-5. The function call
that will cause the stack to be arranged as is shown in the figure below
has four arguments. From left to right these arguments are xlen,
*xdata, ylen, and *ydata. The lengths are simple character
values, and the pointers in this case must be 20-bit values. If a function
call to this routine is compiled with the small memory model, or any
other model with the exception of the compact model, the stacking
will include the extension registers as shown below.

Stack Contents

SP after XK is saved
X after tsx XK SP after pushm x, d
X
D SP after function call
SP at rts
CCR
PC SP before function call
*xdatak
*xdata
ylen
*ydatak
*ydata

Figure 6-5: Stacking Sequence That Passes Extension Registers
The B register, which is the righthand side of the D register, will
contain the left-most parameter when the function is called. There­
fore, the parameter xlen will be found in the least significant bits of
the location labeled D in the above stack outline. The routine shown
below will make use of both the X and the Y registers. The compiler
allows for all registers with the exception of the X register to be