Page 75 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

52 Chapter 2 The Instruction Set

arithmetic type of instruction. For example, suppose that one wants to look at the
contents of some memory location, say $811, and branch to location L if the contents of
location 458, treated as a signed number, are greater than 0, The sequence
TST $811
BGT L

does exactly this. If the TST instruction had left V unaffected, we would have had to use
the longer sequence:
LDAA $ 811
CMPA #0
BGT L

A little more experience will show that the designer's choice here is quite reasonable,
because we will find a more frequent use of signed branches for load instructions than for
checking for signed overflow, as we will do in the next chapter.

Do You Know These Terms?

See the end of Chapter I for instructions.

stack subroutine hardware interrupt service routine
push jump to I/O interrupt latency time
pull subroutine interrupt interrupt inhibit
hardware stack return address handling interrupt mask
buffer branch to interrupt handler stop disable
post byte subroutine device handler machine state
hidden register return from interrupt service return from
prefix byte subroutine routine interrupt