EET 2261 Unit 6
The Stack; Subroutines

Read Almy, Chapter 11.

Homework #6 and Lab #6 due next week.

Quiz next week.
The Stack
•The stack is a section of RAM used for
temporary storage.
•The term “LIFO”
(last-in-first-out) is
often used to
describe the stack.
•The last value you
put in is the first one
you’ll take out, as in
a plate dispenser at a
salad bar.
The Stack Pointer
•The programmer gets to decide where in
RAM the stack is located. She does this
by setting the value in the stack pointer
register (SP).
•This value is treated as the address of
the “top” of the stack.
•Usually the address used is the highest RAM
address, plus one. For us, that address is
$4000.
•So in many programs, one of the first
instructions will be LDS #$4000.
Review: HCS12 Memory Map
•See page 26 of
Device User
Guide.
Pushing and Pulling
•We use the word “push” for putting values
onto the stack, and the word “pull” for
retrieving values from the stack.
•As we push or pull, the stack pointer (SP) is
automatically updated to reflect the fact that the
top of the stack has moved:
•Each push decrements SP by 1 or 2.
•Each pull increments SP by 1 or 2.
•See next slide for push and pull instructions.
Push Instructions and Pull
Instructions
(Table from p. 82 of the HCS12 CPU Reference Manual.)
Using the Stack: Example
•Predict contents after each instruction
executes:
Instruction
A
B
SP
Mem
$3FFD
Mem.
$3FFE
Mem.
$3FFF
LDS #$4000
0
0
$4000
0
0
0
LDAA #$12
$12
0
$4000
0
0
0
PSHA
$12
0
$3FFF
0
0
$12
DECA
$11
0
$3FFF
0
0
$12
LDAB #$34
$11
$34
$3FFF
0
0
$12
PSHD
$11
$34
$3FFD
$11
$34
$12
0
0
$3FFD
$11
$34
$12
PULD
$11
$34
$3FFF
$11
$34
$12
PULA
$12
$34
$4000
$11
$34
$12
LDD #$00
Other Uses of the Stack
•As seen above, the programmer can use the
stack by issuing push or pull instructions.
•Also, under certain conditions the HCS12
automatically pushes data onto the stack or
pulls data off the stack. No push or pull
instructions are needed in these cases.
•Examples: Subroutines (which we’ll study in a few
slides) and interrupts (which we’ll study in a few
weeks) are the two most common conditions of this
sort.
Initializing the Stack Pointer
•Since the HCS12 automatically uses the stack
under certain conditions, it’s a good idea to
initialize the stack pointer (with an LDS
instruction) at the beginning of every program,
even if you don’t plan to use any push or pull
instructions.
•Otherwise, the stack could be located
anywhere in memory, possibly in an area of
memory that overlaps with your program’s code
or data.
Avoiding Conflicts with the Stack
•Be sure to locate the top of the stack far away
from the location of your code and data.
Otherwise your code and data may be wiped
out by stack operations.
•If we always start our programs at $2000 and
locate the top of the stack at $4000, that leaves
plenty of room between our code and the stack.
•Question: How many bytes (and kilobytes)
are there between addresses $2000 and
$4000?
Subroutines
•A subroutine is a small program that performs
some well-defined task within a bigger
program.
•Why use subroutines?
•If you have a block of code that must be
executed at different places within your
program, make it a subroutine.
•Long, complicated programs are easier to
understand and maintain if you break the
program up into separate chunks
(subroutines).
Calling a Subroutine
•
The first instruction in a subroutine has a
label, which serves as that subroutine’s
name.
•
You can call a subroutine by name using
either of these instructions:
• Branch to Subroutine (BSR), which uses
relative addressing mode.
• Jump to Subroutine (JSR), which uses
direct, extended, or indexed addressing
mode.
Returning from a Subroutine
•
A subroutine must end with the RTS (Return
from Subroutine) instruction, which causes
execution of the calling program to resume.
Main Program
.
.
.
BSR Subby
ADDA $1000
Subroutine
Subby: LDAB #05
.
.
.
RTS
.
.
.
•
BSR is similar to BRA, except that it also causes the
processor to “remember” where to resume when it hits a RTS.
Subroutines and the Stack
•
How does the HCS12 remember where in
the main program to resume when it hits a
RTS?
•
When it executes the BSR instruction, it
pushes the Program Counter (which
contains the address of the next instruction)
onto the stack.
•
When it hits the RTS, it pulls this address
back from the stack into the Program
Counter. This lets it pick up where it left off
in the main program.
No Need For You To Push or Pull
•
Recall from earlier that the HCS12 has
several push and pull instructions (such as
PSHA and PULA) that let you place values
on the stack or remove them from the stack.
•
You don’t need to use these instructions
when you branch to a subroutine. The
HCS12 pushes and pulls the desired values
automatically.
Jump and Subroutine Instructions
(Table from p. 79 of
the HCS12 CPU
Reference Manual.)
•
CALL and RTC are advanced instructions that we won’t use.
CALL is similar to BSR, and RTC is similar to RTS.
Overall Program Structure
•
Many programs have a general structure that
contains three major parts:
1. Initialization code that executes once
when the program starts.
2. The program’s main body, which typically
is a “forever” loop that starts with a label
and ends with a BRA to that label. The
main body will often contain one or more
BSR instructions that call subroutines.
3. Subroutines, each of which begins with a
label and ends with an RTS.
Overall Program Structure:
Example
ABSENTRY Entry
ORG $2000
Entry:
CLI
LDS #$4000
MainLoop: BSR Sub1
BSR Sub2
BRA MainLoop
Sub1:
Sub2:
INCA
RTS
INCB
RTS
END
Initialization
Main body
(“forever”
loop)
Subroutines
Troubleshooting Programs with
Subroutines
Single Step (F11). Steps one
instruction at a time.
Step Over (F10). For most
instructions it’s the same as Single
Step, but for BSR it executes the
entire subroutine as one step.