Beginners hardware programming questions.

Hi RiscOS users, I am thinking of buying a Raspberry Pi 3 to run RISC OS Open and do a spot of programming. I never had an Archimedes but a long time ago I had a Beeb and used to enjoy low level programming. I fancy learning ARM assembly language, via the BASIC assembler, and thought Risc OS maybe the way to go. I would also like to “program” the underlying hardware, not use the operating system calls. The initial coding is for my learning experience and not meant for a wide audience so compatibility is irrelevant. I have a few questions that hopefully someone could answer.

1. Is it possible to get direct access to the screen memory, the region above HIMEM in BBC basic?
2. Is there the equivalent of the BBC 6845 where you can set up the screen memory base address and manipulate it to perform horizontal/vertical scrolling?
3. Can you “trap” the screen vertical/horizontal sync?
4. Is there the equivalent of the BBC 6522 timer functionality, ie load the timer and an interrupt is generated when the timer expires?
5. Is there the equivalent to the BBC Advanced User Guide which details hardware programming?

Sorry if these questions don’t make any sense in this brave new world.

Many thanks,
Gary.

In answer to your questions:

1. Yes its possible to directly access screen memory
2. Yes, you can set the screen base address although screen memory doesn’t wrap, so triple buffering is the preferred method
3. An IRQ is generated at VSync and there’s an Event triggered for programs to hang off of. Hsync isn’t accessible. VSync however doesn’t occur at the end of the scan, on a Pi it’s around half way down the scan, so its only useful if you’re triple buffering
4. Yes, there are timers although on the Pi you need to use the HAL timer for precision events. 100Hz is the best you’ll get without delving into the HAL
5. No, there’s no details on a lot of this stuff, although there’s a lot of knowledgeable folk on here that can help you figure it out

A note on question #5. The specs of the original BBC Micro’s hardware were pretty much set in stone. RISC OS runs on a variety of different ARM based hardware, so there is no straightforward guide to manipulating specific hardware in RISC OS. However the datasheets for the various platforms are available.
Depending on what you want to do, in many cases it’s not that hard to access the hardware directly and manipulate it.

Although it’s not a great bit of code, you could check out my Lib RO GPIO which talks to the GPIO hardware directly.
It doesn’t do things the RISC OS way via the HAL or modules. It just maps a chunk of IO memory space and gives access to it. Terrible for security, stability etc. etc.
You don’t need to use the library. It’s more of a usable example on how to meddle with the hardware with absolute impunity.