The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 13
Listing 13-6. This assembly-language routine is similar to Listing 13-1,
except that external interrupt 1 causes the routine to run.
;On external interrupt 1, Port 1, bit 0 is complemented
org 4013h ;vector for external
;interrupt 1
cpl p1.0 ;complement Port 1, bit 0
;(pin 1)
pop psw ;push psw was automatic on
;interrupt,
;but pop psw must be added
reti
end
and add this to the end, before returning to BASIC-52:
pop psw ;restore program status word
Your routine can then write to registers R0-R7 without worrying about conflicting with
BASIC.
Interrupts
BASIC-52 also includes a way of adding assembly-language routines that respond to
interrupts. Normally, the 8052 stores its interrupt vectors (the locations where the program
jumps on interrupts) from 03h to 2Bh in code memory. Since these locations are in ROM in
the 8052-BASIC, your programs can’t change their contents. But built into BASIC-52 is the
ability to place alternate interrupt routines from 4003h to 402Bh.
To illustrate, Listing 13-6 is an assembly-language interrupt routine. The routine’s origin is
4013h, which is BASIC-52’s alternate vector for external interrupt 1. The interrupt routine
has the same function as Listing 13-1. It toggles bit 0 of Port 1, then returns to BASIC-52.
These are a few things of note about Listing 13-6:
• You must have code memory at 4013h, since BASIC-52 specifies that this location
must contain either the interrupt routine or a jump to a longer routine. If the routine is
longer than 8 bytes, use a jump instruction, such as sjmp 4033h, to prevent overwriting
any interrupt vectors that follow.
232 The Microcontroller Idea Book