![]() Furthermore, if a jump/branch is executed, the value in the PC register is updated accordingly. Depending upon the case, it is either incremented more than once or not at all for that clock cycle. Exceptions to this are when the current instruction is 4 bytes (2 pages) long or takes more than a single clock cycle to execute. Normally, the value in the PC register is incremented after executing the instruction it currently points to, which is typically once every clock cycle. Note that the Program Counter also stores the address of a “page” instead of a byte. The Program Counter is also called the Instruction Pointer in some other architectures and they both mean the same thing. The Program Counter in AVR is 16 bits wide in most cases, and 22 bits wide on certain microcontrollers with bigger flash sizes. ![]() A register is a hardware unit that stores a fixed length number as long as power is supplied to it. This is where your microcontroller starts reading and executing instructions from when it is powered on.Īt any given moment, the address of the instruction being executed is stored in the Program Counter (PC) register. In a similar way to this, the instructions that make up your program are written to the flash in contiguous locations, starting from the address/location 0x00. ![]() After this, each pair of characters would then be taken and written to the first available location in the following manner. We then get the ASCII values of each of the characters, which are 0x48, 0圆5, 0圆C, 0圆C, 0圆F, and 0x00 respectively. Do not confuse this with the NULL byte added to the end of C/C++ strings. In most cases, a NULL suffices (“Hello” becomes “Hello\x00”). If they are of an odd length, the assembler adds an extra byte (usually 0/NULL) at the end of it.įor example, if we want to write the string “Hello” to the flash, we would first have to modify the string to make its length even i.e. This implies that any string literals/arrays which are stored in the flash must have an even length (in bytes). This means that when uploading programs to/reading a program from the microcontroller’s flash, the smallest unit which can be accessed is a page and individual bytes can not be accessed this way. Since Instructions on the AVR architecture are encoded into either 2 or 4 bytes, the flash is “paged” into 2-byte pages while it is being accessed by any external hardware or when executing jump/branch operations. Each location in the flash has an address (usually written in hexadecimal notation), which is used when reading from or writing to the flash. its contents are preserved even when the microcontroller is powered down. The Flash memory of your microcontroller is where your program is stored. If you wish to read more about the AVR architecture and its hardware components, then click here. Block Diagram of AVR microcontrollersįor the purpose of this blog, we will look at the Flash and SRAM in a little more detail with respect to the role they play in the execution of your program. Shown below is a block diagram of AVR microcontrollers. Because of this, it helps to be aware of the hardware components inside an AVR microcontroller and the role some of them play in the storage, execution, and management of your program. While writing Assembly, however, you must deal with the hardware in a much more direct manner. High-level languages (such as C/C++) provide you with constructs that your microcontroller does not inherently understand. Let’s get started! Program Storage and Execution in the AVR Architecture You will get the most out of this blog, if you are familiar with C/C++ microcontroller programming, either within the Arduino ecosystem (Arduino IDE) or using the tool-chain that Atmel/Microchip provides (Atmel Studio). The blog has been divided into sections to help in selective reading as well. This is a long blog because, unlike several other tutorials that cover some aspect or the other (and do it well), the attempt in this one is to take you through the complete journey from the very basics of assembly language to writing a working example, building it and seeing it run on your setup. ![]() We will put it all together with an example program to blink an LED. We will see how a program is stored and executed by the Microcontroller’s hardware the syntax of the Assembly language, and also how to build and upload your program using the toolchain provided by Atmel/Microchip. In this blog, I will be explaining how to get started with bare-metal Assembly Language programming on AVR microcontrollers, along with an example for the ATmega328P.
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