I am doing an ongoing project to write a simplified OS for hobby/learning purposes. I can generate hex files, and now I want to write a script on the chip to accept them via serial, load them into RAM, then execute them. For simplicity I'm writing in assembly so that all of the startup code is up to me. Where do I start here? I know that the hex file format is well documented, but is it as simple as reading the headers for each line, aligning the addresses, then putting the data into RAM and jumping to the address? It sounds like I need a lot more than that, but this is a problem that most people don't even try to solve. Any help would be great.
1 Answers
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way too vague, there are many different file formats and at least two really popular ones that use text with the data in hex. So not really helping us here.
writing a script on chip means you have an operating system running on your microcontroller? what operating system is it and what does the command line look like, etc.
assembly is not required to completely control everything (basically baremetal) can use asm to bootstrap C and then the rest in C, not a problem.
Are you wanting to download to ram and run or wanting to download and then burn to flash to reset into in some way?
Basically what you are making is a bootloader. And yes we write bootloaders all the time, one for each platform, sometimes borrowing code from a prior platform sometimes not.
First off on your development computer, windows, mac, linux, whatever, write a program (in C or Pascal ideally, something you can easily port to the microcontroller) that reads the whole file into an array, then write some code that basically accepts one byte at a time like you would if you were receiving it serially. Parse through that file format whatever format you choose (initially, then perhaps change formats if you decide you no longer like it) take real programs that you have built which the disassembler or other tools should have other output options to show you what bytes or words should be landing at what addresses. Parse this data, printf out the address/byte or address/word items you find, and then compare that to what the toolchain showed. carve the parsing tool out and replace the printf with write to memory at that address. and then yes you jump to the entry point if you can figure that out and/or you as the designer decide all programs must have a specific entry point.
Intel hex and motorola s-record are good choices (-O ihex or -O srec, my current favorite is --srec-forceS3 -O srec), both are documented at wikipedia. and at least the gnu tools will produce both, you can use a dumb terminal program (minicom) to dump the file into your microcontroller and hopefully parse and write to ram as it comes in. If you cant handle that flow you might think of a raw binary (-O binary) and implement an xmodem receiver in your bootloader.
