AVRvsARM (1).pdf

AVRvsARM (1).pdf - 1 Nathan Wilson(z3287546 COMP 2121...

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20/04/2011 1 Nathan Wilson (z3287546) COMP 2121 Assignment One: Comparing the ISA of ARM and AVR 1. Overview ARM microprocessors are known for being very small and cheap, simple compared to most other general purpose processors, while still retaining adequate performance. They have a very simple hardware design with a very small die. Some of the more salient features of ARM microprocessors are: Low power consumption. Power consumption can be controlled easily, thanks to the small die and simple pipeline design. High modularity. Components such as caches, floating point and other co-processors are all optional. This means that developers can make highly specialised microprocessors by only including the minimum of components needed. Conditional execution. After doing a compare, rather than branching there are instructions such as ADDNE (add if not equal) that only execute if the previous compare satisfies the required condition. It allows for very compact code. Powerful memory transfer instructions. Included in the instruction set are instructions allowing you to load/store up to 16 registers at once. This allows for faster, more compact code involving memory access. There are a number of differences between the AVR and ARM architectures, which can be divided broadly into the categories of Memory models, Registers, Instruction set, and Data types. 2. Memory models ARM processors have a single memory map, with both code and data sections stored in a single memory space. This is also known as a ‘Von Neumann’ memory architecture. In ARM processors this space has a maximum size of (2^32 -1) bytes, about 4GB. It is a little-endian memory and addressed in four byte long words. That is, it is aligned on four byte boundaries, with the two least significant bits being zero. Registers for I/O devices are mapped into this memory space as well, while the general registers are usually implemented in a co-processor. Some ARM Cores, such as the Cortex- M0/1/3, do not have a co-processor and instead do in fact need to implement the general registers in memory. Along with this memory space, there are also a number of caches on the chip, depending on the type of ARM Core and the developer’s customisations. Cache memory is small, very fast memory used to retain copies of recently used memory values. This enables faster access than it would if the processor was accessing the memory every time. 3. Registers AVR has 32 8-bit general registers. ARM also has 32 general-purpose registers, but they are 32-bit registers and only 16 are available for use in user mode (One of which is the program counter). The other 16 are used for system level programming and exception handling, and only become available when the processor is running in the corresponding operating mode.
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