I had a bit of a leap backwards when I discovered the STM32F103 doesn't take ARM instructions, it's all THUMB-2 and relative, icky pooh, so I'm now writing in C.

I had a bit of a leap forward when I discovered the port bit set reset register GPIOx->BSRR

It's a 32 bit register for a 16 bit port, set a bit in the top half and you set a pin, set a bit in the bottom half and you reset the pin. Fiendishly useful if you are sharing the port with other peripherals you don't want to touch.

The port control registers come in Low and High GPIOx->CRL GPIOx->CRH with 4 bits per port which is conveniently one digit in Hex so you can see what you are getting in the way of inputs and outputs at a glance. For example:-

GPIOB->CRL = 0x22222222; Gets you 8 push pull outputs b0-7 on Port B

Much easier to see what is going on a glance.

I have a rudimentary 4 axis stepper written, now I need to get data in to it. I've bought a 4Gb micro SD card. If I can dump the entire cut into it then I can stop worrying about drip feeding and comms errors, just have it send reports on a timer tick so I can do fancy graphics on the PC. Watch out, SD cards come with a "Class" number which is the transfer rate in MHz, if you don't see a Class number assume it's slow, I got a Class 4.