AVR Assembler
1
BRANCHES SUBROUTINES AND MEMORY
USAGE
M. Neil - Microprocessor Course
Outline
2
 Memory on the AVR processor
 Addressing modes for accessing memory
 Program Flow: Subroutines and the stack
 Memory Test Program
 Program Design
 The LED display program
M. Neil - Microprocessor Course
3
Memory in the
AVR
There are Several
different types of
memory on the AVR
which you can
use/access
Flash Program Memory
Data SRAM (internal
and external)
Registers
EEPROM
M. Neil - Microprocessor Course
Program Memory
 Organized as 64K 16bit words


Addressed $0000-$FFFF
128 Kilo-Bytes of memory
 This is where the program
which is executing is stored

The Program Counter (PC)
holds the address in program
memory of the current
instruction
 Data can be stored in program
memory

The data stored in program
memory needs to be copied
into data memory before it can
be used – there is a special
instruction (LPM) for doing
this
M. Neil - Microprocessor Course
4
Data Memory
 There is some internal
memory (SRAM) on the
Atmega128 which can be used
for storing data (and also the
Stack)


Internal SRAM memory is 4K
X 8 bits (4 Kilo-Bytes $0100$10FF)
The registers and I/O ports
occupy the lower part of this
memory (Addresses $0000$00FF)
 There is also the possibility of
external memory being
installed


Not on our boards
Can add if your project needs
M. Neil - Microprocessor Course
5
Registers
6
 The registers actually
occupy the lowest 32
bytes of internal SRAM.
 Most registers can be
used generally for 8 bit
operations
 R16-R31 can be used
with the ldi instruction
 R26-r31 can also be used
as the X/Y/Z address
registers
M. Neil - Microprocessor Course
Accessing memory – Addressing modes
7
neg r20
R20  $00-r20
add r20,r24
R20  r20+r24
M. Neil - Microprocessor Course
Addressing Input/Output memory
8
 There are 6 I/O ports
 PORTA-PORTG
 Each has

PINX


DDRX


Input Port
Data Direction Register
PORTX

Output Port
M. Neil - Microprocessor Course
in r20,PIND
R20  PIND
out PORTB,r16
PORTB  r16
Working with SRAM Data
9
 We can define addresses
where we want to store
variables, and move data in
and out of the SRAM with
the lds,sts instructions
M. Neil - Microprocessor Course
.equ myVariable = $0208
lds r16, myVariable
sbi r16, $22
sts myVariable, r16
Using the X, Y, Z registers to address SRAM
10

The registers r26-r31 are special and can be
used to store an address (16 bits $0000$FFFF) which points to the SRAM




With these registers, any location in SRAM
can be read from or written to


r26:r27 is XL:XH
r28:r29 is YL:YH
r30:r31 is ZL:ZH
ld r16,Y
There are also pre/post
increment/decrement modes to make it
quicker to address arrays of data


ld r3,Z+
ld r7,–X
.equ myVariable = $0208
ldi XL, low(myVariable)
ldi XH, high(myVariable)
ld r16,X
sbi r16, $22
st X,r16
M. Neil - Microprocessor Course
;load XL(r26):XH(r27) with
;address of myVariable
;copy data from SRAM
;subtract 34 from the byte
;store result in SRAM
Program Memory and LPM - a picture
11
Using the Z register to
point at program memory
Program Memory
Z Register
LPM will load
Address
Bits 8-15
Bits 0-7
$200
$B1
$100
$B2
$B1
$201
$B2
$101
$B4
$B3
$202
$B3
$102
$B6
$B5
$203
$B4
$103
$B8
$B7
$204
$B5
$205
$B6
M. Neil - Microprocessor Course
We multiply the address by 2 to select
which 16 bit word
The Least Significant Bit (LSB) is then
used to select which byte to read with
the LPM instruction
Copying data from PM to SRAM
12
Can use only the Z register to copy data from PM to
a register. Auto increment modes make more
compact code – Use the special lpm instruction
.equ
ldi
ldi
ldi
ldi
ldi
myArray=$0800
ZH,high(myTable*2)
ZL,low(myTable*2)
XH,high(myArray)
XL,low(myArray)
r16,22
;
;
;
;
;
;
Address in SRAM
Load the Z register with the
address of data in PM
Load X register with the
address in SRAM
there are 22 bytes to copy
lpm
st
dec
brne
ret
r17,Z+
X+,r17
r16
loop
;
;
;
;
move one byte from PM to r17
store in SRAM (and increment)
count down bytes copied
keep going until finished
.db
“This is just some data”
loop:
myTable:
M. Neil - Microprocessor Course
More details about Program memory
13
 The Z register is only 16 bits
wide


Can address 64K Bytes
There are 128K Bytes of flash
program memory
 The LPM instruction
addresses the lower 64K Bytes
 The RAMPZ register allows
access to the upper 64K Bytes
with the ELPM instruction
; **** RAMPZ Setup Code ****
ldi r16,$00
; 1 = ELPM acts on upper 64K
out RAMPZ,r16
; 0 = ELPM acts on lower 64K
M. Neil - Microprocessor Course
Storing data in program memory
14
 You can create initial values for data in your program code
 These will be stored in the program memory


Accessed using the labels you define
See the examples below for the assembler directives needed to create
data in the
 You can copy them into SRAM (as we did on the previous
page) or use them directly from the program memory

Access will be quicker from SRAM and values can be changed
MyByteTable:
.DB $00,$01,$02,$03,$04,$05,$06,$07;Table loading
MyWordTable:
.DW $0908,$0B0A,$0D0C,$0F0E,$1110 ; Load 5 16-bit
.DW $1514,$1617,$1918,$1B1A,$1D1C ; Next 5 16-bit
MyStringTable:
.DB “abcdefghijklmnopqrstuvwxyz” ; Line of ascii
M. Neil - Microprocessor Course
with 8 bytes
words in the table
words in the table
characters
15
Sometimes we need to
work with numbers larger
than 8 bits
There are Word
operations which can be
used to perform 16 bit
operations on the upper 4
register pairs
(25:24,27:26,29:28,
31:30)
adiw - add immediate to
word
longloop:
ldi XH,high($DEAD)
ldi XL,low($DEAD)
loop:
sbiw XH:XL,1
X
brne loop
ret
;put large number
;in X register
;subtract 1 from
;continue till 0
sbiw – subtract
immediate to
word
We can also use the carry
bits for longer numbers –
this requires some coding
adc – add with carry
sbc – subtract with carry
M. Neil - Microprocessor Course
Using 16 Bit Counters
Program Flow, subroutines, local variables
16
 You will want to break your program up into small
pieces or subroutines
 You will need to set up loops to repeat small pieces of
program
 A special register called the stack pointer is used to
call subroutines, and temporarily store registers
when using subroutines
 The simulator can help you understand what is
happening in your program as well
M. Neil - Microprocessor Course
17
SRAM usage
The on-board RAM must
hold the Stack Pointer
The Stack Pointer keeps
track of where the
program execution has
been
Allows temporary storage
of registers
SRAM also holds data
variables
Typical to have Stack
grow downwards and
data grow upwards
Must avoid stack/data
collisions!
M. Neil - Microprocessor Course
; ***** Stack Pointer Setup Code
ldi r16, $10
; Stack Pointer Setup
out SPH,r16
; Stack Pointer High Byte
ldi r16, $FF
; Stack Pointer Setup
out SPL,r16
; Stack Pointer Low Byte
18
Subroutines:
call,rcall,icall
call to a small unit of
code which does a given
task
•
•
rcall mysub
• PC+1 stored on
stack
• PC is changed to
mysub:
ret
• PC is popped
from the stack
Good Practice to
push/pop any registers
used in the subroutine
push r24 – stores r24
register byte at the
location in the Stack
Pointer (SP) then
decrements the SP
pop r24 – loads r24
with the byte pointed to
by SP then increments
the SP
Must call ret at the end
of the subroutine
M. Neil - Microprocessor Course
main:
wait:
loop:
rcall wait
rjmp main
; call our subroutine
push r24
ldi r24,$FF
dec r24
brne loop
pop r24
ret
;
;
;
;
;
preserve r24
count down bytes
keep going until finished
restore r24
19
Functions:
Call to a small unit if
code which does a given
task
Can depend on input
parameters – passed to
the function in registers
or on the stack
Good Practice to
push/pop any registers
used in the subroutine
Must call ret at the end
of the subroutine
Result stored in a
register
M. Neil - Microprocessor Course
main:
ldi r24,3
rcall mult10
out PORTB,r24
rjmp main
mult10: push r25
push r26
ldi r25,$0A
ldi r26,$00
loop:
add r26,r24
dec r25
brne loop
mov r24,r26
pop r26
pop r25
ret
; input parameter in r24
; call our subroutine
; put the answer on the LED
;
;
;
;
;
preserve r25
preserve r26
we will multiply by 10
sum initialized to 0
add r24 to the sum
;
;
;
;
;
keep going until finished
put sum in r24
restore r26
restore r25
return with answer in r24
Stack Pointer – Examples, Call
20
22: 0e 94 7f 00
call
0xfe
26: 97 01
movw r18, r14
…..
;
; our subroutine
;
fe: ef 92
push
r14
; do some things – and use r14
; for whatever we want
100:7c 01
movw r14, r24
;
13e: ef 90
pop
r14
140:08 95
ret
M. Neil - Microprocessor Course
PC
Location Data
$22
$10FF
00
$10FE
00
$10FD
00
$10FC
00
$10FB
00
$10FA
00
$10F9
00
$10F8
00
$10F7
00
SPH
SPL
r14
$10
$FF
$99
Stack Pointer – Examples, Call
21
22: 0e 94 7f 00
call
0xfe
26: 97 01
movw r18, r14
…..
;
; our subroutine
;
fe: ef 92
push
r14
; do some things – and use r14
; for whatever we want
100:7c 01
movw r14, r24
;
13e: ef 90
pop
r14
140:08 95
ret
M. Neil - Microprocessor Course
PC
Location Data
$FE
$10FF
00
$10FE
26
$10FD
00
$10FC
00
$10FB
00
$10FA
00
$10F9
00
$10F8
00
$10F7
00
SPH
SPL
r14
$10
$FD
$99
Stack Pointer – Examples, Call
22
22: 0e 94 7f 00
call
0xfe
26: 97 01
movw r18, r14
…..
;
; our subroutine
;
fe: ef 92
push
r14
; do some things – and use r14
; for whatever we want
100:7c 01
movw r14, r24
;
13e: ef 90
pop
r14
140:08 95
ret
M. Neil - Microprocessor Course
PC
Location Data
$100
$10FF
00
$10FE
26
$10FD
99
$10FC
00
$10FB
00
$10FA
00
$10F9
00
$10F8
00
$10F7
00
SPH
SPL
r14
$10
$FC
$99
Stack Pointer – Examples, Call
23
22: 0e 94 7f 00
call
0xfe
26: 97 01
movw r18, r14
…..
;
; our subroutine
;
fe: ef 92
push
r14
; do some things – and use r14
; for whatever we want
100:7c 01
movw r14, r24
;
13e: ef 90
pop
r14
140:08 95
ret
M. Neil - Microprocessor Course
PC
Location Data
$102
$10FF
00
$10FE
26
$10FD
99
$10FC
00
$10FB
00
$10FA
00
$10F9
00
$10F8
00
$10F7
00
SPH
SPL
r14
$10
$FC
$xx
Stack Pointer – Examples, Call
24
22: 0e 94 7f 00
call
0xfe
26: 97 01
movw r18, r14
…..
;
; our subroutine
;
fe: ef 92
push
r14
; do some things – and use r14
; for whatever we want
100:7c 01
movw r14, r24
;
13e: ef 90
pop
r14
140:08 95
ret
M. Neil - Microprocessor Course
PC
Location Data
$13E
$10FF
00
$10FE
26
$10FD
99
$10FC
00
$10FB
00
$10FA
00
$10F9
00
$10F8
00
$10F7
00
SPH
SPL
r14
$10
$FC
$xx
Stack Pointer – Examples, Call
25
22: 0e 94 7f 00
call
0xfe
26: 97 01
movw r18, r14
…..
;
; our subroutine
;
fe: ef 92
push
r14
; do some things – and use r14
; for whatever we want
100:7c 01
movw r14, r24
;
13e: ef 90
pop
r14
140:08 95
ret
M. Neil - Microprocessor Course
PC
Location Data
$140
$10FF
00
$10FE
26
$10FD
99
$10FC
00
$10FB
00
$10FA
00
$10F9
00
$10F8
00
$10F7
00
SPH
SPL
r14
$10
$FD
$99
Stack Pointer – Examples, Call
26
22: 0e 94 7f 00
call
0xfe
26: 97 01
movw r18, r14
…..
;
; our subroutine
;
fe: ef 92
push
r14
; do some things – and use r14
; for whatever we want
100:7c 01
movw r14, r24
;
13e: ef 90
pop
r14
140:08 95
ret
M. Neil - Microprocessor Course
PC
Location Data
$26
$10FF
00
$10FE
26
$10FD
99
$10FC
00
$10FB
00
$10FA
00
$10F9
00
$10F8
00
$10F7
00
SPH
SPL
r14
$10
$FF
$99
Exercises:
27
 Exercise 1 : Last time you made a counter. Now
use a subroutine to delay the speed of the counter.
 Exercise 2 : You can delay further by cascading
delay subroutines.
 Exercise 3 : Memory Test Program: Write a
program that writes (st) incrementing numbers to
the SRAM. Read (ld) them back, compare with
what you expect to see and if it is not correct send a
pattern of lights to PORTB.
 What happens if you start writing before address
$0100
M. Neil - Microprocessor Course
LED sequence display
28
 Write a program that puts the contents of a table
into program memory and then reads the contents
back sequentially, byte by byte, and outputs them
to the LEDs on PORTB.

Chose the bytes so that they will make a pattern when they
are displayed on the LEDs
 Use delays to slow down the program so you can
see the different patterns flashing as they light the
LEDS on PORTB.


Allow the user to change the speed of flashing using the
switches on PORTD
Allow the user to start/stop the pattern using switches on
PORTD
M. Neil - Microprocessor Course
Structured Programming
29
•
Design your programme into a main control
segment, which calls sub-routines
•
•
marks will be given on the modularity of your code
Before starting to write code make a simple design diagram
outlining the tasks which need to be done
•
•
Take 5-10 minutes, and show this to your demonstrator
Use a Top Down Modular Programming approach to design
Subroutines should perform specific tasks or
repeated functions
• Put lots of comments in your code
•
M. Neil - Microprocessor Course
Top Down Modular Programming
30
time
Calculator
Initialise
Reset
Load
values
Prompt
message
Output
character
M. Neil - Microprocessor Course
Get
command
Input
Input
character
Calculate
Interpret
command
Output
character
Select/run
routine
Output
Format
output
Display
Output
character
c
o
m
p
l
e
x
i
t
y