The document discusses assembler programming for the Atmega328P microcontroller. It describes how to store fixed data in flash memory using the DB and DW directives and how to determine word and byte addresses. It also explains the HIGH() and LOW() functions, which return the high and low bytes of a 16-bit value and are used to initialize the Z register before loading data from flash memory. Arithmetic and logical expressions that can be used in assembler programming are also presented.

1. Assembler Programming of Atmega328P
(Lecture-11)
R S Ananda Murthy
Associate Professor and Head
Department of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,
Mysore 570 006
R S Ananda Murthy Assembler Programming of Atmega328P

2. Storing Data in Flash Memory
Though flash memory is referred to as program memory,
we can also store fixed data in it using DB and DW
directives as shown above.
Since each word address in flash stores two bytes in
little-endian order, at any word address, the low-byte
address is twice the word address and the high-byte
address is low-byte address+1.
Determine the word and byte addresses in the example given
above.
R S Ananda Murthy Assembler Programming of Atmega328P

3. HIGH() and LOW() Functions
AVR assembler provides these two functions.
HIGH() returns the higher byte of a 16-bit value.
LOW() returns the lower byte of a 16-bit value.
These functions are typically used to initialize Z register to
point at data bytes stored in the flash memory before using
the LPM instruction.
For example:
LDI ZH,HIGH($100<<1);Initialize ZH.
LDI ZL,LOW($100<<1) ;Initialize ZL.
LPM
R S Ananda Murthy Assembler Programming of Atmega328P

4. Arithmetic and Logical Expressions in ALP
Arithmetic Operators Logical Operators
Symbol Operation Symbol Operations
+ Addition & Bitwise AND
− Subtraction | Bitwise OR
* Multiplication ^ Bitwise XOR
/ Division ~ Bitwise NOT
% Modulo << k Shifts left by k bits
>> k Shifts right by k bits
We can write arithmetic and logical operations in ALP using the
symbols given above.
R S Ananda Murthy Assembler Programming of Atmega328P

5. Examples of Arithmetic and Logical Expressions
R S Ananda Murthy Assembler Programming of Atmega328P

6. Device Definitions File
This file contains .EQU directives which define labels
assigned to various ports, port bits, registers and register
bits.
For Atmega328P device, this file is named as
m328pdef.inc
When we select a device for an assembly language project
in Atmel Studio, these definitions are available to the user
program through the device definitions file.
If we are not using Atmel Studio, then, we have to use
.INCLUDE directive to include the Device Definitions File in
the .asm file to use the standard labels.
R S Ananda Murthy Assembler Programming of Atmega328P

7. A Glimpse of Device Definitions File for Atmega328P
R S Ananda Murthy Assembler Programming of Atmega328P

8. Labels for Status Flags in m328Pdef.inc File
In m328Pdef.inc file, labels for status flags are defined as
shown above.
R S Ananda Murthy Assembler Programming of Atmega328P

9. Setting Specific Flags in SREG using Flag Labels
Labels for status flags can be used to set specific flags as
shown above.
R S Ananda Murthy Assembler Programming of Atmega328P

10. Labels Related to Memories in m328Pdef.inc File
We can use RAMEND to initialize Stack Pointer as shown later.
R S Ananda Murthy Assembler Programming of Atmega328P

11. Stack in Atmega328P
Stack is a portion of R/W memory which is used for storing
temporary data, for storing local variables and for storing
return addresses before executing subroutines or Interrupt
Service Routines.
In AVR MCUs, stack is, implemented as Last-In-First-Out
(LIFO), growing from higher memory to lower memory
locations.
PUSH Rr command will store the content of Rr in the stack
and then decrements the SP by 1.
POP Rd command will first increment the SP by 1 and
then, byte from the stack is copied to Rd.
R S Ananda Murthy Assembler Programming of Atmega328P

12. Stack Pointer in Atmega328P
In Atmega328P SP is a 16-bit register having two 8-bit
registers SPH and SPL in the I/O space of the R/W
memory in MCU as shown above.
In AVR MCUs having very small memory only SPL will be
present.
The Stack Pointer (SP) register always points to the top of
the stack.
R S Ananda Murthy Assembler Programming of Atmega328P

13. License
This work is licensed under a
Creative Commons Attribution 4.0 International License.
R S Ananda Murthy Assembler Programming of Atmega328P