PDP-11 memory question

Jerome Fine adds the following: (a) My experience with upgrading the backplane is VERY satisfactory. It was done by someone with sufficient skills with a soldering iron. And yes, wire-wrap wire was used. The specific model was the VT103 which was a 4 * quad backplane, i.e. 4 quad boards or 8 dual boards. And terminating resistors did not seem to be needed - perhaps because of so few boards. (b) With regard to the use of the M8044 and M8045 memory boards, at one time I did that mod as well - using only 4096 bytes for the IOPAGE addresses (from 170000 to 177776). However, if I remember correctly (perhaps Megan can help with this aspect), it ONLY worked with either the RT11SJ or RT11FB monitors - i.e. UNMAPPED. As soon as I use the 11/23 board WITH the MMU activated, the MMU hardware under RT11XM forced the full 8192 bytes of the IOPAGE to be used - there was no other option. In addition, with the 11/73, it was NOT possible to use a 4096 Byte IOPAGE AT ALL Comments on the use of the 4096 Byte IOPAGE vs an 8192 Byte IOPAGE: If you have ONLY an 11/03 which can't run RT11XM or is running a stand-alone application without any operating system, then using a 4096 Byte IOPAGE does provide the user with an extra 4096 Bytes of program address space. HOWEVER, these days if anyone has at least an 11/23 and is running at least RT-11, then using RT11XM with at least 128 KBytes of memory does not seem to be a factor as far as cost is concerned. As such, I have yet to see an application which runs under RT11SJ and conforms to normal RT-11 rules which can't run under RT11XM, although I am certain that someone could immediately find one for me. But in most cases, the problems with an ISR (Interrupt Service Routine) is simply to place the code into the address range 00000 to 17776 (PAR0 which allows up to almost 8192 Bytes). So even if the program itself must have access to the IOPAGE, it is still possible to use extended memory for the address range 020000 to 157776 (PAR1,2,3,4,5,6) which means that the program has much more memory available than even under RT11SJ since now the locations used by the Resident Monitor in RT11SJ are also available and more than compensate for losing the address range from 160000 to 167776 for the RT11SJ Resident Monitor when the 4096 Byte IOPAGE is used. If memory mapping is OK, all 7 of the upper PAR ranges can be used under different windows, as required. I have one program which makes use of 152 KBytes of upper memory - 2 buffers of 48 KBytes each and initial set-up and data retention of 56 KBytes - the IOPAGE is not used. What is interesting is that I/O requests can be initiated in either of the 48 KByte buffers (.ReadC EMT Request) immediately after which the window for the other 48 KByte buffer can be activated and the I/O will automatically continue into the now unmapped 48 KByte buffer while the program processes the 96 blocks in the other 48 KByte buffer - using the code in PAR0 and PAR1 (00000 - 37776). The very nice part is that this program initially requests ONLY 8192 Bytes of memory and then checks to see if it is running under RT11XM or not. If the latter, a .SetTop is done and the rest of the program is read into memory if there is sufficient available and it uses as much as is available for the now very small buffers. Otherwise, if the former, 152 KBytes of memory are requested and 2 buffers of 48 Kbytes each are then available plus 56 Kbytes of code and data to manage everything. Now that is impossible with an 11/03 and even an 11/23 running the RT11SJ monitor.