Jerome Fine replies: I suspect that having memory above 160000 is very special and that such must be allowed for in a different manner from normal RAM. In respect of a 2Kbyte I/O page, I hope you (and I am sure you probably are - just a slip of the fingers) are referring to 8192 Bytes as being the normal I/O page (160000 -> 177777) in a 64 KByte address space. The 2 KWord or 4096 Byte I/O page (170000 -> 177777) is allowed ONLY for older CPUs like the LSI-11/03 and the PDP-11/23 WHEN the MMU unit is turned OFF. Some of the older memory boards had a jumper or a wire wrap setting just for this situation. So you could run either RT11SJ or RT11FB and get and extra 4 KBytes or 2 KWords for the user program. In almost all cases, RT11SJ was used since that monitor is smaller than RT11FB and therefore allows extra space. NOTE that for the LSI-11/03 and a PDP-11/23 (without an MMU chip which had to run RT11SJ), this was the only solution to obtain the most memory for the background job. In addition, if the background job was very unorthodox and did things like ISRs (Interrupt Service Routines - if you don't know what they are don't bother) at non-RT11XM compatible addresses (I ran across this situation last year with a program that could not run under RT11XM - remember that 4 KByte I/O page Megan), then a small I/O page under RT11SJ would indeed be all that could be done. HOWEVER, this solution was actually self-defeating in most cases. Of course it required a system with an MMU unit in the case of an 11/23 AND probably at least 128 KBytes of memory. AND for the J11 chip systems (11/73 etc.), running under RT11XM actually allows far more memory than even RT11SJ. Note also that no J11 chips allow a 4 KByte I/O page (as far as I could determine and I think that Megan found the same result) in any case! Only an 8192 Byte I/O page is allowed. For anyone who wants a hint, under RT11XM (and now even more so under RT11ZM when DATA SPACE is allowed in addition to INSTRUCTION SPACE - also allowed with recent versions of TSX-PLUS) it is possible to have ISRs in the first 8192 bytes of memory and request extended memory for the address space where the resident monitor is located in Kernel Space and even use User Space addresses for where the I/O page addresses are located if the I/O page is not required. For example, I know of a version of BINCOM which initially does a LOAD into only the first 8192 bytes of memory (000000 -> 017776). If RT11FB is running, the rest of the program is loaded in the normal manner. If RT11XM is running, the address space 020000 -> 177777) is mapped into extended memory (if sufficient is available) providing a full 64 KBytes to run the program and provide buffers for directories and data. THEN, to put the icing on the cake, TWO more areas of memory are requested (to use double buffering 040000 and when the next pair of files needs to be determined and opened, the area above 040000 with the original program can be activated and used. Altogether, 152 KBytes just in extended memory are used compared to a maximum of about 48 KBytes that are available to the background program under RT11SJ. Sincerely yours, Jerome Fine RT-11/TSX-PLUS User/Addict