Editing PDP-10 (section)

===KL10===
[[File:PDP-10 1090.jpg|thumb|KL10-DA 1090 [[CPU]] and 6 Memory Modules]]
The original KL10 PDP-10 (also marketed as DECsystem-10) models (1080, 1088, etc.) use the original PDP-10 memory bus, with external memory modules. Module in this context meant a cabinet, dimensions roughly (WxHxD) 30 x 75 x 30 in. with a capacity of 32 to 256 kWords of [[magnetic-core memory]]. The processors used in the [[DECSYSTEM-20]] (2040, 2050, 2060, 2065), commonly but incorrectly called "KL20", use internal memory, mounted in the same cabinet as the [[Central processing unit|CPU]]. The 10xx models also have different packaging; they come in the original tall PDP-10 cabinets, rather than the short ones used later on for the DECSYSTEM-20.  The differences between the 10xx and 20xx models were primarily which operating system they ran, either TOPS-10 or [[TOPS-20]]. Apart from that, differences are more cosmetic than real; some 10xx systems have "20-style" internal memory and I/O, and some 20xx systems have "10-style" external memory and an I/O bus.  In particular, all ARPAnet TOPS-20 systems had an I/O bus because the AN20 [[Interface Message Processor|IMP]] interface was an I/O bus device.  Both could run either TOPS-10 or TOPS-20 microcode and thus the corresponding operating system.

====Model B====
The later Model B version of the 2060 processors removes the 256 [[binary prefix|kilo]][[Word (data type)|word]] limit on the virtual address space by supporting up to 32 "sections" of up to 256 kilowords each, along with substantial changes to the instruction set.  The two versions are effectively different CPUs.  The first operating system that takes advantage of the Model B's capabilities is TOPS-20 release 3, and user mode extended addressing is offered in TOPS-20 release 4.  TOPS-20 versions after release 4.1 only run on a Model B.

TOPS-10 versions 7.02 and 7.03 also use extended addressing when run on a 1090 (or 1091) Model B processor running TOPS-20 microcode.

====MCA25====
The final upgrade to the KL10 was the MCA25 upgrade of a 2060 to 2065 (or a 1091 to 1095), which gave some performance increases for programs which run in multiple sections.

====Massbus====
The I/O architecture of the 20xx series KL machines is based on a DEC bus design called the [[Massbus]]. While many attributed the success of the PDP-11 to DEC's decision to make the PDP-11 Unibus an open architecture, DEC reverted to prior philosophy with the KL, making Massbus both unique and proprietary. Consequently, there were no aftermarket peripheral manufacturers who made devices for the Massbus, and DEC chose to price their own Massbus devices, notably the RP06 disk drive, at a substantial premium above comparable IBM-compatible devices. [[CompuServe]] for one, designed its own alternative disk controller that could operate on the Massbus, but connect to IBM style 3330 disk subsystems.

====Front-end processors====
[[File:KL10-front-end.jpg|thumb|KL10 frontend PDP-11/40]]
The KL class machines have a PDP-11/40 [[front-end processor]] for system start-up and monitoring. The PDP-11 is booted from a dual-ported RP06 disk drive (or alternatively from an 8" [[floppy disk]] drive or [[DECtape]]), and then commands can be given to the PDP-11 to start the main processor, which is typically booted from the same RP06 disk drive as the PDP-11. The PDP-11 performs watchdog functions once the main processor is running.

Communication with IBM mainframes, including [[Remote job entry|Remote Job Entry]] (RJE), was accomplished via a DN61 or DN-64 front-end processor, using a PDP-11/40 or PDP-11/34a.<ref>{{Cite web|url=http://www.inwap.com/pdp10/usenet/decnet|title=USENET alt.sys.pdp10 postings about PDP-10 front end processors}}</ref><ref>{{cite web|url=http://pdp-10.trailing-edge.com/BB-J724A-SM_1980/01/documentation/dn60m.man.html|title=DN60 Maintenance Manual|id=JBS-77-001-02-U|date=October 31, 1978|publisher=Digital Equipment Corporation}}</ref>