So the SAMS manual has better schematic, but what it come down to is the OSI decodes the upper address pages into separate signals, which are then directed to a 8 line priority encoder, which turns it back into addresses for the ROM. The signals are jumpered to map a particular memory page to a particular ROM page address using the U15 "socket". EXCEPT the 74148 output is inverted! So input "0" generates address '7' on A0-A2 outputs of the 74148.
So if you want to map FF00 to page 4 in the ROM, jumper U15 pin3 to pin 10 etc.
(I used a similar setup on the 505 board to switch between BASIC-In-ROM and Disk boot by swapping the FFxx jumper setting on a SPDT switch.)
[Don't forget to set the chip select jumpers to work with EPROM, as the mask ROMS OSI used had different select requirements.]
The picture below shows the mapping, with the red addresses shown in non-inverted (logical) form.
- OSI502 ROM select
- 502sel.jpg (127.46 KiB) Viewed 7048 times
I wonder where OSI documented this?
U15 OSI "Socket" pinout
1 = FDxx (out)
2 = FExx (out)
3 = FFxx (out)
4 = Gnd
5 = Gnd
6 = Gnd
7 = ROM Select Page 7 (in) (A0=1, A1=1, A2=1 which goes to A8,A9,A10)
8 = ROM page 6
9 = ROM page 5
10 = ROM page 4
11 = ROM page 3
12 = ROM page 2
13 = ROM page 1
14 = ROM page 0
As shown in SAMS picture:
FFxx selects page 4
FExx selects page 3
FDxx selects page 2
which matches the SYNMON layout.
SYNMON page layout
- Page 0 FE00 OSI 440 board 65V monitor (ASCII KB)
Page 1 FF00 OSI 440 board C/W/M BASIC boot (ASCII KB)
Page 2 *FD00 C2/540 Polled Keyboard routine
Page 3 *FE00 C2/540 65V Monitor
Page 4 *FF00 C2/540 BASIC Boot (C/W/M?)
Page 5 FD00 initializes HDisk controller (CD74/CD36 winchester HD)
Page 6 FE00/FF00 OSI 65A Serial Monitor
Page 7 *FF00 C2/540 disk boot (H/D/M?) works with serial or video
Although you could use SYN600 with some adjustments...
SYN600 - page layout
- page 000 'H/D/M' maps to $FF00 for a C2/C4 540Vid disk system
page 100 keypoller maps to $FD00 for a C2/C4 540Vid system
page 200 monitor maps to $FE00 for a C2/C4 540Vid system
page 300 'C/W/M' maps to $FF00 for a C2/C4 540Vid tape system
page 400 disk boot maps to $FC00 for a C1 system
page 500 keypoller maps to $FD00 for a C1 system
page 600 monitor maps to $FE00 for a C1 system
page 700 'D/C/W/M' maps to $FF00 for a C1 system
Or anything in your own custom ROM!
So it looks like to map FFxx to page 7 you need to jumper U15 3 to 7, and to map FExx to page 6 U15 pin 2 to pin 8.
(Let us know if it actually works this way !)
Hope this helps!