UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Gemini 80-Bus System

JohnBHanson · 7th Oct 2020, 5:16 pm

Port e4 is control for the floppy disk and has bits for drive/density etc.

The 829 and 849 are different. In particular the 829 uses the bottom 4 bits for drive select so one should be selected eg 01 for a:, 02 for b:, 04 for c: 08 for d:.

The 849 uses the bits in a different manner bit 2 (value 04) selects the connector and the bottom 2 bits select the drive. For compatibility for boot roms/boot sectors the 849 is wired so that value 1 selects a: same as 829.

The 11h write to 0ffh is setting up the memory paging for the nascom system, That
supports 4 pages, 4 bits for write enable and 4 bits for read enable. Only one page
can be setup for read at any one time.

for gm813 you will also have to setup the 4k memory paging before this. See hdboot.mac for details.

7th Oct 2020, 5:16 pm	#429
JohnBHanson Heptode Join Date: Aug 2009 Location: Worthing, Sussex, UK. Posts: 661	Re: Gemini 80-Bus System Port e4 is control for the floppy disk and has bits for drive/density etc. The 829 and 849 are different. In particular the 829 uses the bottom 4 bits for drive select so one should be selected eg 01 for a:, 02 for b:, 04 for c: 08 for d:. The 849 uses the bits in a different manner bit 2 (value 04) selects the connector and the bottom 2 bits select the drive. For compatibility for boot roms/boot sectors the 849 is wired so that value 1 selects a: same as 829. The 11h write to 0ffh is setting up the memory paging for the nascom system, That supports 4 pages, 4 bits for write enable and 4 bits for read enable. Only one page can be setup for read at any one time. for gm813 you will also have to setup the 4k memory paging before this. See hdboot.mac for details.