16 May
2021
16 May
'21
6:56 a.m.
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?
Rob,
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.
--Bjoren