> And there's the rub. It's got to be modern and powerful for anyone to be
> interesting. But if it's modern and powerful, it's going to be far too
> complex to tinker with at the level you would with the C64. If you have
> a C64, it's *feasible* to write a short machine code routine which
> performs a key-scan and pokes the video hardware in response to
> different key presses. You could, given a month of Sundays,
> realistically build your own micro-OS.
>
> *Nobody* is ever going to do that for a Raspberry Pi. It takes something
> as complex as Linux to power it.

Download RiscOS for it - far simpler than Linux (no protection or 
multi-user) and very simple to write assembler within the built-in BASIC 
interpreter. By default the stuff you write will run as a single-task 
you have complete control over the machine. Also the whole GUI is 
ridiculously fast and responsive as it was originally designed for ARM 
processors under 50 MHz (the web browser is way faster than whatever the 
one included in raspbian, although not as feature-rich).

> Similarly, with the C64 you can write a few POKE commands and watch the
> screen turn green or hear a sound play or something.

You can issue poke commands from BASIC directly in RiscOS - IIRC ?<addr> 
is a variable you can use to read or write a byte to that address, or 
!<addr> to read/write a 4-byte word in one go. RiscOS BASIC also allows 
you to drop into the assembler very simply and then obviously reading 
and writing to memory is easily possible. The assembly language is 
pretty easy to learn.

The version of RiscOS for the pi also contains new software interrupts 
(that are easily called from BASIC) to read and write to the GPIO pins 
on the PCB. The following program will output a square wave on one of 
the pins (apparently at about 300 kHz).

REPEAT
SYS "GPIO_WriteData" ,17,1
SYS "GPIO_WriteData" ,17,0
UNTIL FALSE

> The GPU on the
> Raspberry Pi are closed-source; you need to sign an NDA just to see what
> registers it has! Nobody is going to be experimenting with that anytime
> soon.

AFAIK they haven't managed to get GPU access in RiscOS yet. If you 
wanted to do GPU stuff then you'd be better off with Linux, for example 
I use mine mainly with xbmc as a media centre, plays 1080p videos fine 
from an external drive through HDMI to a TV, so the GPU must be involved 
somewhere.

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