[wiml]

If you want something you can almost fully understand, I'd go another step or two lower, to a microcontroller like an AVR or one of the old Motorola chips (68HC11 or something). These chips were actually designed by hand and expected to be programmed by hand, and their documentation reflects this. They also ave much less hidden microarchitectural state than a modern CPU.

Once you're familiar with that move to a more modern microcontroller like an Arm Cortex-M0, and after that maybe something with off-chip memory, a MMU, etc.

[LeifCarrotson]

Those 8-bit controllers with (exclusively) on-chip SRAM and Flash are indeed quite simple - they can be summed up as "Jump to 0x0000", but really are surprisingly complex once you get into bootloaders and interrupt service routines and so on.

The proprietary blobs and GPU on the Raspberry Pi make basically impossible to have a full understanding of what's happening. Instead, I'd recommend learning with the TI Beaglebone Black, which has an ARM Cortex-A8 with an MMU and lots of open documentation.