Doesn't matter. There's no interleaving, dual-channel, or anything else that would affect speed depending on the number of modules installed. However, if you only want 32 MB and can use 4x 8 MB, use these. Larger modules may come handy later for a build that needs more memory.

Thanks! I have so much RAM it doesn't really matter which ones I use. I was just wondering if there was any difference in performance.

EDIT: Actually, the problem solved itself. The board I'm using doesn't accept 8MB modules in slots 3 and 4, only in slots 1 and 2. So I am using two 16MB modules instead. Cheers!

Sometimes with less modules installed you can use tighter memory timings.

starcube wrote on 2020-03-01, 16:37:

[...]

EDIT: Actually, the problem solved itself. The board I'm using doesn't accept 8MB modules in slots 3 and 4, only in slots 1 and 2. So I am using two 16MB modules instead. Cheers!

Right, and that is why it does matter 😮

8MB modules are double-sided, with either 16 1Mx4 chips or 4 1Mx16 chips. That 'double sided' doesn't mean that there are chips on both sides (there usually are), but it mainly refers to the fact that one of those SIMMs is the equivalent of two single-sided SIMMs. Count the number of data lines (the sum of the number behind the x), single-sided is 32, double-sided 64.

16MB modules are single sided, usually with 8 4Mx4 chips or sometimes (rarely) 2 4Mx16 chips - in any event 32 data lines.

Now, memory controllers can only handle a given number of banks. Exactly how banks are organised depends on the motherboard/chipset, but assuming a late 486 it's simple: 1 single-sided SIMM occupies 1 bank. Most chipsets will support max 4 banks, meaning you can have four single-sided SIMMs, two double-sided SIMMs, or a combination of two single-sided and one double-sided SIMM. In Pentium chipsets, two SIMMs paired form a bank, but similar limits can and do frequently apply there as well. Note that in that case two single-sided SIMMs can be paired with a single double-sided SIMM with twice the capacity; they really are equivalent.

As for performance, that depends on a lot of factors, mainly determined by the memory controller in the motherboard chipset. Some do do bank interleaving, in which case you want to have the banks to interleave with. Others don't, in which case it nominally makes no difference. Tip: next time post the board you're using, helps us be specific. Bus load can be (marginally) significant when overclocking, so you might clock higher with two single sided SIMMs than with four double sided ones - but that's hypothetical unless you are doing unspeakable things to old systems 😉

Finally, two single-sided SIMMs leave room for expansion, where four double-sided ones don't (and even two double-sided SIMMs would max out whatever you are using here).

Anonymous Coward wrote on 2020-03-01, 18:12:

Sometimes with less modules installed you can use tighter memory timings.

No, since the organisation of the modules matters. With two double-bank memory modules you can not use any tighter memory timings than you could with 4 single-bank memory modules. Or in other words: the number of banks used is important for timing, not the number of modules (which is not the same!). Not that it's relevant, as most 486 mainboards won't let you set the timings anyway.

dionb wrote on 2020-03-01, 18:18:

8MB modules are double-sided, with either 16 1Mx4 chips or 4 1Mx16 chips.

You mean double-banked. And 8 MB modules are not generally double-banked. Actually, double-banked was not used that often for smaller sizes and most modules are single-banked.

Thank you for the interesting discussion. The board is a 486 PCI board based on the ALi 1487/1489 chipset. I had no issues using the same four 8MB modules on a board with the UMC8881/8886 chipset, so I guess it's just a chipset thing. If it matters, the 8MB modules are 4Mx16 (each module has four 1Mx16 chips). The manual confirms the board only likes 8MB modules in slots 1 and 2. Empty slots don't matter since I will never be installing more than 32MB in this system.

derSammler wrote on 2020-03-01, 19:16:

[...]

You mean double-banked.

"Double-banked" only applies if one SIMM = one bank. As we didn't know that for sure, I used the more generic term - in fact the only correct one to use when discussing the SIMMs themselves, as opposed to their logical organization when installed in a system.

And 8 MB modules are not generally double-banked. Actually, double-banked was not used that often for smaller sizes and most modules are single-banked.

8MB wasn't a "smaller size" in 486 days... In fact it's quite simple: as a general rule, even two-powers (1MB, 4MB, 16MB, 64MB) will be single-sided and odd two-powers (2MB, 8MB, 32MB, 128MB) will be double-sided. So 8MB modules will almost always be double-sided.

Case in point:

starcube wrote on 2020-03-01, 20:25:

Thank you for the interesting discussion. The board is a 486 PCI board based on the ALi 1487/1489 chipset. I had no issues using the same four 8MB modules on a board with the UMC8881/8886 chipset, so I guess it's just a chipset thing. If it matters, the 8MB modules are 4Mx16 (each module has four 1Mx16 chips). The manual confirms the board only likes 8MB modules in slots 1 and 2. Empty slots don't matter since I will never be installing more than 32MB in this system.

4 chips with 16 data lines = 64 data lines = double-sided.

In fact hit the same limitation today when trying to upgrade my UM8498F-based board to >=32MB. It happily takes double-sided 8MB SIMMs, but only two at a time. Unfortunately I don't have any >=16MB FP modules and UM8498F most definitely doesn't support EDO (tried, got all the crashes you'd expect).

dionb wrote on 2020-03-01, 20:39:

In fact it's quite simple: as a general rule, even two-powers (1MB, 4MB, 16MB, 64MB) will be single-sided and odd two-powers (2MB, 8MB, 32MB, 128MB) will be double-sided.

I think this is the most important piece of information to me in this thread. Thank you for the info!

starcube wrote on 2020-03-01, 21:04:

dionb wrote on 2020-03-01, 20:39:

In fact it's quite simple: as a general rule, even two-powers (1MB, 4MB, 16MB, 64MB) will be single-sided and odd two-powers (2MB, 8MB, 32MB, 128MB) will be double-sided.

I think this is the most important piece of information to me in this thread. Thank you for the info!

Slight nuance:
This 'rule' only holds true for FP and EDO. SDRAM development was more incremental, so odd two-powers were also used. You can have single-sided 64MB and 256MB, but also 128MB DIMMs, and moving into DDR territory, single-sided 512MB and (with DDR2) 2GB DIMMs were also a thing.

dionb wrote on 2020-03-02, 09:03:

starcube wrote on 2020-03-01, 21:04:

dionb wrote on 2020-03-01, 20:39:

In fact it's quite simple: as a general rule, even two-powers (1MB, 4MB, 16MB, 64MB) will be single-sided and odd two-powers (2MB, 8MB, 32MB, 128MB) will be double-sided.

I think this is the most important piece of information to me in this thread. Thank you for the info!

Slight nuance:
This 'rule' only holds true for FP and EDO. SDRAM development was more incremental, so odd two-powers were also used. You can have single-sided 64MB and 256MB, but also 128MB DIMMs, and moving into DDR territory, single-sided 512MB and (with DDR2) 2GB DIMMs were also a thing.

I know how it works with newer memory types. I was specifically asking about a 72-pin SIMM-based system which accepts both FPM and EDO DRAM.