Documentation for F11 Chipset?

Bjoren Davis bdweb at
Sun May 16 08:56:44 CDT 2021

> I have been looking at this and I think you are right. But the reason is
> odd. It looks like the ROMs are never being selected by the ROM address
> decode. I can't find on the printset anything that says what the boot
> address would be, perhaps that is burned into the F11 chipset? However, from
> the Pro technical manual the ROM addresses are in the ranges
> 17730000-17767776, so I think the top 7 bits of the address should all be
> 1s. It looks like I never get anything other than 0s, when the address
> strobe (CT6 RCV AS H on the printset) is asserted. There is activity on the
> F11 chips, so I think they are working.
> Any ideas anyone?

The start address on the DEC Pro is physical address 017760000. As a 
virtual addresses this is at the beginning of the I/O page (0160000). 
This mapping extends up 4 KiB (to physical 017767777 or virtual 0167777).

But, of course, the ROM is actually 16 KiB long.  So where are the other 
12 KiB?

They're at physical addresses 017730000..017757777.  These physical 
addresses are not mapped into virtual address space at reset, but the 
boot ROM does map them during its execution to exactly where you'd 
expect: 0130000..0157777.

Now, just to top off this confusion, the mapping of CPU-perspective 
physical addresses to ROM address lines is a little odd.

It's really best described a table, which I hope doesn't get mangled by 
email formatting (all values in octal):

Virtual     Physical      ROM offset
0130000*    017730000     030000
0137777*    017737777     037777
0140000*    017740000     000000
0147777*    017747777     007777
0150000*    017750000     010000
0157777*    017757777     017777
0160000+    017760000     020000
0167777+    017767777     027777

* = mapped later by boot ROM into virtual address space
+ = low half of I/O page -- mapped at CPU reset time

So you can see the low 14 bits of physical address are fed directly to 
the ROM.  It makes for a slightly lumpy looking layout.

The decode for this is on page CT10 of the schematic.  You can see the 
"ROM ADDRESS DECODER" section which has a NAND of address lines 21..15 
being used as an enable on a 3-to-8 negative-output decoder and a 
3-input negative-input OR on outputs 3,4,5.  This selects physical 
addresses 0177[3,4,5]XXXX.  Then E114 decodes the I/O page locations 
when A12 is low (017760000..017767777). This is the crucial reset-time 
ROM selection decoder.

As to why the CPU starts at 0160000...I swear I saw that once in the 
documentation somewhere but I can't immediately find it again.  I 
believed that the CPU is presented with some kind of word at reset time 
that tells it where to start executing.  I believe that you can see this 
word constructed by E3 and half of E17 on page CT2 of the schematic, but 
I can't find the documentation that describes the layout of the word.  
You can see the word would be 0b11100000LXXXX010 where L is ~(CT2 LPOK 1 
L) and X is undefined.  Notice that the high bits decode to 0160000, and 
I think that's where the start address comes from.

I hope that answers your question.


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