This post comes in the wake up some discussions i had with few people working the embedded domain a few days back.
The question was, where exactly does our code in the micro controller run? The answer again unfortunately is not so straight forward. The reason is because different micro controllers behave in different ways. Let us therefore try to understand how the whole process works so that we can judge it better.
Frankly, the micro controllers at its core are just a bunch of registers on which a few mathematical operations can be done by another piece of hardware ( part of the core) called the ALU(Arithmatic Logic Unit). Most of the micro's thus have a Accumalator(Acc). This is like the mother register and most of the operations (mathematical or logical) will use the Acc register (There are instructions that do not involve the Acc also. like mov H,L does not involve the Acc). To ensure that some instrucution is executed, it must be first fetched, decoded and only there can be some action. To fetch an instruction, it should be readily available. Readily is a fuzzy word here because what is readily on 8Mhz system is really too slow on a 1GHz processor. The readiness depends on how fast we can read the said instrucution. The simplest micro controllers like 8051 keep the instructions in their flash memories. These are accessed by the core and then executed. Some industry folks call this as "Executing from the Flash".
Modern micro's have the concept of cache. This means they will pre-fetch and keep some of the instructions in a faster re-writeable memory. The core thus will use the cache to read the instructions instead of reading from the flash. Crude and simpler implementations of cache are in the form of instruction queue etc. These don't have the capabilities of the cache memory but never the less helps in making execution faster. So, now the instructions are stored in flash, but executed really out of cache memory.
Some micro controllers also provide a wonderful feature where in the instructions can be stored in the RAM. This is generally done while reprogramming. More about reprogramming will be there in future posts. Here the real program resides in the flash memory but one can copy the program to RAM and then set the core to fetch the instructions from the RAM instead of the flash.
Some folks call this "Execution from RAM".
The fact is that most modern micro's can read instructions from both Flash, Ram or in some cases even from communication buses (like CAN/FlexRay). The last part is tricky one and will be discussed in some other post ( because i have to get more information on that yet).
My take is this, all code gets executed in the core. All that can vary is where we are fetching it from.
Please leave your comment if you have one. You can subscribe to this blog by using the links under "Subscribe" section.
The question was, where exactly does our code in the micro controller run? The answer again unfortunately is not so straight forward. The reason is because different micro controllers behave in different ways. Let us therefore try to understand how the whole process works so that we can judge it better.
Frankly, the micro controllers at its core are just a bunch of registers on which a few mathematical operations can be done by another piece of hardware ( part of the core) called the ALU(Arithmatic Logic Unit). Most of the micro's thus have a Accumalator(Acc). This is like the mother register and most of the operations (mathematical or logical) will use the Acc register (There are instructions that do not involve the Acc also. like mov H,L does not involve the Acc). To ensure that some instrucution is executed, it must be first fetched, decoded and only there can be some action. To fetch an instruction, it should be readily available. Readily is a fuzzy word here because what is readily on 8Mhz system is really too slow on a 1GHz processor. The readiness depends on how fast we can read the said instrucution. The simplest micro controllers like 8051 keep the instructions in their flash memories. These are accessed by the core and then executed. Some industry folks call this as "Executing from the Flash".
Modern micro's have the concept of cache. This means they will pre-fetch and keep some of the instructions in a faster re-writeable memory. The core thus will use the cache to read the instructions instead of reading from the flash. Crude and simpler implementations of cache are in the form of instruction queue etc. These don't have the capabilities of the cache memory but never the less helps in making execution faster. So, now the instructions are stored in flash, but executed really out of cache memory.
Some micro controllers also provide a wonderful feature where in the instructions can be stored in the RAM. This is generally done while reprogramming. More about reprogramming will be there in future posts. Here the real program resides in the flash memory but one can copy the program to RAM and then set the core to fetch the instructions from the RAM instead of the flash.
Some folks call this "Execution from RAM".
The fact is that most modern micro's can read instructions from both Flash, Ram or in some cases even from communication buses (like CAN/FlexRay). The last part is tricky one and will be discussed in some other post ( because i have to get more information on that yet).
My take is this, all code gets executed in the core. All that can vary is where we are fetching it from.
Please leave your comment if you have one. You can subscribe to this blog by using the links under "Subscribe" section.
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