The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Calling Assembly-language Routines
Avoiding Program Crashes
It’s very easy to write an assembly-language program that crashes the system and forces you
to reboot. To prevent this, you have to take care that your routines do not interfere with each
other, or with BASIC-52. Remember that BASIC-52 is a program in itself, and it uses many
of the registers and other memory locations, both inside and outside of the 8052, for its own
purposes.
For example, BASIC-52 uses locations 13h and 14h in internal RAM to store the starting
address of the current BASIC program in external RAM. If you overwrite these values,
BASIC-52 will no longer be able to find your program.
The BASIC-52 programming manuals list the registers and other memory addresses used
by BASIC. In general, you should avoid writing to these locations, unless you know what
you’re doing and how to deal with the results.
Often, an assembly-language routine will alter some of the 8052’s registers. You are
responsible for seeing that all critical values are unchanged when the routine returns control
to the program that called it, whether it’s BASIC-52 or another assembly-language program.
The stack is a convenient way to preserve values on entering a routine, and to restore them
on exiting. The stack is a special area of memory with a last-in, first-out structure, which
means that you read values from the stack in the reverse order that you wrote them. Storing
values in the stack area is called pushing, or placing, values on the stack. Retrieving values
from the stack area is called popping them off the stack.
Assembly language has push and pop instructions for accessing the stack. (BASIC-52’s
PUSH and POP instructions access a separate area called the argument stack.)
You can also preserve values by selecting a unique register bank for use by a routine. The
8052 has 32 registers arranged in four banks of eight, from 0 to 1Fh in internal data memory.
You can access the registers by specifying the address, or by selecting a register bank and
specifying a register from R0 to R7 within the bank. For example, if bank 0 is selected, R0
is location 00h, but if bank 1 is selected, R0 is location 08h.
BASIC-52 uses banks 0, 1, and 2, but uses bank 3 only with the PGM instruction, so this
bank is usually free for other uses. Bits 3 and 4 of the 8052’s program status word (psw)
select the register bank. When you call an assembly-language routine, BASIC-52 automatically
selects register bank 0. To select bank 3, add this to the beginning of your routine:
push psw ;save program status word
orl psw,#18h ;select register bank 3
The Microcontroller Idea Book 231