pdp 11 boot rom 23-E39A9

Jerome Fine replies: That 'most intelligent" controller (MSCP) was supposed to be needed to handle large disk drives. The HD: interface (under Ersatz-11) has the capability of handling 2 TB disk drives and is probably the "dumbest" (perhaps cleanest and most easily implemented would be better) controller that I have ever had the pleasure of writing code for. The MSCP interface seems to be much more complex than is actually needed. Since I have been successful in producing a working device driver based on the HD: interface used in Ersatz-11 for a 22 bit Qbus emulation under RT11XM, I have also discovered just how much additional overhead must be present with MSCP code vs the "dumb" HD: interface. As for the size of the code needed to support MSCP, I haven't, as yet, started to use a translation table which DU(X).SYS supports, but the single RT-11 partition version which I first developed to replace VMX.SYS (which has a LOADed code footprint of just 87 words) is considerably smaller and much faster (3 times the speed of VM: and 2 times the speed of DU:), mostly because far fewer instructions need to be executed in the device driver for an I/O transfer. Note that the original version of the HD: device driver John Wilson wrote in 1995 worked well with unmapped monitors (RT11SJ and RT11FB), but since it did not support 22 bit Qbus usage, it did not function very well using PDP-11/73 emulation when I was using RT11XM running under Ersatz-11. A post that I just made recently compared how long it takes to copy an RT-11 MSCP 32 MB partitions vs an RT-11 HD: 32 MB partitions. Since the overhead should have been averaged out over many blocks per read / write request and the actual transfer of data been about the same, I was initially VERY surprised to observe that an MSCP copy took much more than twice as long or much more than 2 seconds vs just about 1 second per 32 MB copy using the HD: device driver. On the other hand, even the overhead of using an EMT request in RT-11 must be substantial since when subroutine code that is in user space (i.e. does not use an EMT request) is used, the speed of a transfer is again twice as fast as using the device driver. It takes only about 0.5 seconds to copy a 32 MB RT-11 partition using subroutines in user space. Based on the code in the MSCP device driver in RT-11, it is very obvious that MSCP code does MUCH more than in the HD: device driver. Under MSCP, there is also MUCH more code (I am finally responding directly to Dan's observation) that needs to be executed. On the other hand, under HD: all that is required is to transfer the word count, block number and buffer address to the IOPAGE registers (well the physical unit number as well which actually takes more instructions than any of the other variables since the 4 bits of the unit number are also in the CSR) and initiate the transfer. So the reason that Dan notes that the MSCP interface needed so much more code than the other so-called "dumb" controllers seems (in my opinion) because MSCP is just far more complex than it needs to be to accomplish the same result that HD: manages to produce with much less code and much less overhead. The statement that MSCP is an intelligent controller and that the other controllers are "dumb" is, therefore, perhaps correct in that the MSCP interface is able to handle very large disk drives. But the unnecessary overhead of MSCP that compares with the same capability of managing disk drives of up to 2 TB with HD: seems to indicate that a simple dumb interface can be much more efficient as well as easier to use - and take much less code to implement and time to execute. Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.