scommstech
Experienced Member
- Joined
- Jul 16, 2007
- Messages
- 69
Hi
I'm trying to check out an old XT motherboard and have hit a problem.
Does anybody understand the memory refresh process that IBM's XTs
used.
The board uses the 4164 DRAM therfore there are 64000 memory cells
per chip.
The XT uses RAS/CAS and 16 bit addressing. I would expect RAS to use 8
Add bits 0-7 = 256, and CAS to use Add bits 8-15 = 256 thus giving a total
of 64000 memory cells.
The refresh only uses RAS every 2ms set up by the DMA. All 4 banks get RAS
at the same time. I believe that toggling RAS causes a cell refresh.
For all the 64000 cells to be refreshed each RAS toggle must have
an accompanying incrementing address 0 to 256. So 256 RAS pulses would
refresh to whole memory.
Using a logic analyzer on the faulty board the RAS pulse is accompanied by
a constant address. This I thought was the problem, ie the address not
incrementing. I put the analyzer on a good motherboard and it's the same,
ie constant address.
Where is my theory going wrong ???.
Can anybody throw some light on this.
Regards
Scommstech
I'm trying to check out an old XT motherboard and have hit a problem.
Does anybody understand the memory refresh process that IBM's XTs
used.
The board uses the 4164 DRAM therfore there are 64000 memory cells
per chip.
The XT uses RAS/CAS and 16 bit addressing. I would expect RAS to use 8
Add bits 0-7 = 256, and CAS to use Add bits 8-15 = 256 thus giving a total
of 64000 memory cells.
The refresh only uses RAS every 2ms set up by the DMA. All 4 banks get RAS
at the same time. I believe that toggling RAS causes a cell refresh.
For all the 64000 cells to be refreshed each RAS toggle must have
an accompanying incrementing address 0 to 256. So 256 RAS pulses would
refresh to whole memory.
Using a logic analyzer on the faulty board the RAS pulse is accompanied by
a constant address. This I thought was the problem, ie the address not
incrementing. I put the analyzer on a good motherboard and it's the same,
ie constant address.
Where is my theory going wrong ???.
Can anybody throw some light on this.
Regards
Scommstech