giantclam wrote on 2023-10-15, 01:15:
Do 25ns chips get along with 35ns timings?...I guess you'll find out =)
If the new chips you ordered are actual 25ns chips (and not relabelled 50ns chips), they should work fine.
In case you wonder about the 100MHz marking in them, whereas a period of 25ns would correspond to a frequency of 40MHz, this is because the "25ns" is the RAS access time, so it assumes the chip being in idle state (no bank open) and measures the time from the idle state to getting valid data from that chip. As long as you hit the optimal timing for dealing with the address multiplexing, the /RAS and the /CAS signals, you can get valid data 25ns after you started getting the chip out of "idle" mode (i.e. asserting /RAS). Because this measures the time that passed between asserting /RAS and the data access working, this is called "/RAS access time". At the end of the /RAS access time, the chip is not in idle state, but the bank needs to be closed before a new bank can be opened. The time from asserting /RAS for reading from bank to asserting /RAS again for reading another bank is higher than 25ns and is called the "/RAS cycle time", which on 25ns RAM is likely about 40ns.
On the other hand, the 100MHz marking on that chip indicates a suitable memory clock of the graphics card, if it operates in "singly cycle EDO mode". You Virge card most likely does (if it doesn't the 3D performance is even more atrocious as it generally is on the 86c325), and you are accessing different cells inside a single bank that is already active. The 10ns associated with 100MHz is the time from asserting /CAS for one read cycle until /CAS can be re-asserted for the next read cycle in the same bank. This timing is known as the "/CAS cycle time". 10ns /CAS cycle time is a standard timing for 25ns RAM. The 86c325 is officially specified for clocks around 50 to 55MHz, so it clearly won't use the full potiential of the 10ns /CAS cycle time, but in case you are interested in 3D applications, and you want to run applications at higher resolutions than 400x300, you might want to look into overclocking that chip. I found that given sufficiently fast RAM, 86c325 chips often work fine up to 80MHz, which will give them a severely needed speed boost if your favorite S3d game made the stupid choice to run at 640x480 (as Terminal Velocity did, most likely because their game assets are designed for 320 or 640 pixels horizontal). Which makes me wonder - how much fun could Terminal Velocity have been on a 486 computer, if it included a 320x240 hardware-assisted mode?
