Can I replace a XC68LC040RC33B In an LC575 With A XC68040RC40 MC68040RC40 40MHz CPU?

[mcbeav]

I'm new to 68K macs, and I have an LC575 (in a color classic), and I'm just looking at what things I can do to it. I found a legit 40MHz XC68040RC40 MC68040RC40 CPU for sale, and I'm having a hard time finding info on if I can swap the CPU out for the higher clocked non LC chip, and if so, can I expect it to actually run at 40MHz and expect a performance gain?

 

[finkmac]

yes you can swap it. it will not run faster unless you overclock the machine.
the speed rating are just what that particular part was qualified to run at. i.e. tested and warrantied to run at.

 

[mcbeav]

yes you can swap it. it will not run faster unless you overclock the machine.
the speed rating are just what that particular part was qualified to run at. i.e. tested and warrantied to run at.

That's good to know, thanks for the info. Does the LC variant lack the FPU? Would I theoretically get some type of performance increase by swapping to the new CPU?

 

[finkmac]

correct, the LC variant doesn't have an FPU. You might see a performance increase in apps that use the FPU... I bet mathematica would run better

 

[Melkhior]

You might see a performance increase in apps that use the FPU...

Emphasis on the 'might'. Beware that:

(a) applications that use FP instructions directly just won't run on the LC version
(b) applications that don't use FP at all, or use it via built-in emulation, will not benefit from the FPU in the non-LC
(c) applications that uses SANE, or applications that detect the FPU to choose between hardware FPU and emulation, will benefits from running on the 'full' version of the MC68040 (or having a MC68881 or MC68882 added to a MC68020 or MC68030).

Case (b) is very common, as in those Macintoshes sold with no hardware FPU were extremely common: this includes all those based on the MC68000 and the LC, LCII, LCIII, IIsi, LC475, LC575, Centris 610, etc. ; some of those could get an optional FPU but applications vendor could not rely on it. Some higher-end applications for maths or other sciences could fall under (a), as for a scientific user the FPU could be expected to be present (and was there by default on all higher-end systems such as the II, IIx, SE/30, IIcx, IIci, IIfx, most Quadras, etc.).

So if you don't have an application that refuses to run on your LC575, you probably don't have anything in (a). Remains to be seen if you are using something from (c) that would benefits from the additional FPU. Otherwise, replacing a LC040 by the full 040 won't help you much. And extracting a MC68040 from its socket without damaging anything is not obvious (hence people creating pulling tools). As full MC68040, and in particular higher-rated one, can be expensive, make sure you really need/want the FPU before spending any money on it. For period-accurate software, it's unlikely to be really needed.

OTOH, if you intend to run e.g. NetBSD on it and have a XC68LC040, then go for it. FPU is a lot more needed on NetBSD, and the XC68LC040 has a bug that makes it impossible to trap and emulate FPU instructions (MC68LC040 are OK though, Motorola eventually fixed the bug).

 

[Spaceduel]

I'm new to 68K macs, and I have an LC575 (in a color classic), and I'm just looking at what things I can do to it. I found a legit 40MHz XC68040RC40 MC68040RC40 CPU for sale, and I'm having a hard time finding info on if I can swap the CPU out for the higher clocked non LC chip, and if so, can I expect it to actually run at 40MHz and expect a performance gain?

The answer is really don’t need it. The programmers couldn’t count on it being there because most of the machines were sold without a math coprocessor. So 99% of the software was written to not use one.

 

[mcbeav]

Emphasis on the 'might'. Beware that:

(a) applications that use FP instructions directly just won't run on the LC version
(b) applications that don't use FP at all, or use it via built-in emulation, will not benefit from the FPU in the non-LC
(c) applications that uses SANE, or applications that detect the FPU to choose between hardware FPU and emulation, will benefits from running on the 'full' version of the MC68040 (or having a MC68881 or MC68882 added to a MC68020 or MC68030).

Case (b) is very common, as in those Macintoshes sold with no hardware FPU were extremely common: this includes all those based on the MC68000 and the LC, LCII, LCIII, IIsi, LC475, LC575, Centris 610, etc. ; some of those could get an optional FPU but applications vendor could not rely on it. Some higher-end applications for maths or other sciences could fall under (a), as for a scientific user the FPU could be expected to be present (and was there by default on all higher-end systems such as the II, IIx, SE/30, IIcx, IIci, IIfx, most Quadras, etc.).

So if you don't have an application that refuses to run on your LC575, you probably don't have anything in (a). Remains to be seen if you are using something from (c) that would benefits from the additional FPU. Otherwise, replacing a LC040 by the full 040 won't help you much. And extracting a MC68040 from its socket without damaging anything is not obvious (hence people creating pulling tools). As full MC68040, and in particular higher-rated one, can be expensive, make sure you really need/want the FPU before spending any money on it. For period-accurate software, it's unlikely to be really needed.

OTOH, if you intend to run e.g. NetBSD on it and have a XC68LC040, then go for it. FPU is a lot more needed on NetBSD, and the XC68LC040 has a bug that makes it impossible to trap and emulate FPU instructions (MC68LC040 are OK though, Motorola eventually fixed the bug).

Incredibly informative, and just helped me make my decision. Thank you for the help!