NeXTdimension memory map and hardware dithering

Started by andreas_g, November 15, 2016, 02:10:23 pm

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andreas_g

Hello all,

this question is related to emulating the NeXTdimension board. For complete emulation of the board it is essential to know the memory map and functions of different areas of memory.

This is what we know (or think we know) at the moment.

Devices:

0xF000xxxx:
Video I/O controller

0xFF20xxxx:
RAMDAC

0xFF40xxxx:
Unknown register(s)

0xFF80xxxx:
Memory controller


Memory:

0xF0800000 - ?
0xF2800000 - ?
0xF4800000 - ?
0xF6800000 - ?
Memory areas likely related to dithering and RAM, unknown function

0xF8000000 - 0xFBFFFFFF:
RAM (4 banks of 16 MB each)

0xFD000000 - 0xFD3FFFFFF:
0xFD400000 - 0xFD7FFFFFF:
0xFD800000 - 0xFDBFFFFFF:
0xFDC00000 - 0xFDFFFFFFF:
Memory areas likely related to dithering and VRAM, unknown function

0xFE000000 - 0xFE3FFFFF:
VRAM (4 MB)

0xFE800000 - 0xFEBFFFFF:
Mirror of VRAM, unknown function

0xFF000000 - 0xFF0001FF:
512 bytes of memory likely used as base of hardware dithering

0xFFF00000 - 0xFFFFFFFF:
ROM (128 kB, mirrored 8 times)

Can anyone help to complete the map? Maybe some NeXT engineers are around. For me it would be interesting if the above memory map is complete and if the things we think to know are correct. Furthermore it would be very important to know the function of the yet unknown areas. Especially it would be interesting what is intended to happen when reading/writing to memory likely related to hardware dithering (0xFDxxxxxx, 0xF08xxxxxx, 0xF28xxxxx, 0xF48xxxxx, 0xF68xxxxx, 0xFE8xxxxx, 0xFF000xxx).

Any help is welcome!

Andreas