It has recently come to my attention that there exist two speed jumpers on my AMI Mark V Baby Screamer motherboard, Rev. B. Rev. A boards don't have these. They are J35 and J37. These jumpers are not well documented in my printed manual, dated 11/14/91.
In my manual dated 11/14/91, there no mention of J35, but there is one deceiving note concerning J37 in Appenix A, page 87:
140 MHz Operation 21. For 64K cache, U38 is not required. J37 must be shorted. 3 4The remaining [cache] sockets - U31, U32, U34, U37, U39, U41, U42, U44, U45, U46, and U47 become 16K x 4 12 ns SRAM. 5 62. The 256 KB cache option at 40 MHz is not present because 64K x 4 12 ns SRAM is not yet available.
In the past, I ignored this line because note number 1 sounds like J37 is to be shorted if using 64K cache, however, it may be that J37 is to be shorted if FSB is set to 40 MHz. There are a lot of typos in the manual in general.
So which information is correct?
I ran some benchmarks at 66 MHz and noted that shorting J37 reduces the memory read throughput from 14.8 MB/s to 12.1 MB/s (Cachechk). Moving J35 from 2-3 to 1-2 had no measurable impact. I plan on playing with these two jumpers further to see if I can get 2x40 Mhz to light up, rather than running the board at 2x33.
Plan your life wisely, you'll be dead before you know it.