92 lines
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92 lines
6.1 KiB
Markdown
---
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layout: post
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title: Curious PDP-11 Features
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date: 2016-11-13 13:00:00
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permalink: /blog/2016/11/13/
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---
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Last week, I ran PDPjs through a series of early [PDP-11 Paper Tape Diagnostics](/apps/pdp11/tapes/diags/).
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These tests were originally released in 1970 and are documented in such DEC publications as the
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[MAINDEC USER REFERENCE MANUAL](https://s3-us-west-2.amazonaws.com/archive.pcjs.org/pubs/dec/pdp11/diags/MAINDEC-11-DZQAB-B-D_User_Reference_Manual_Oct73.pdf)
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(October 1973).
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[Tests 1-12](/apps/pdp11/tapes/diags/#tests-1-12) (MAINDEC-11-D0AA-PB through MAINDEC-11-D0LA-PB)
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uncovered a couple of uninteresting bugs in the emulator that affected the ASRB and RORB instructions, both involving some
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incorrect masks. But aside from those hiccups, the tests ran fine. Ditto for [Test 13](/apps/pdp11/tapes/diags/#test-13).
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Things got much more interesting in [Test 14](/apps/pdp11/tapes/diags/#test-14). For starters (and as DEC's documentation
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points out), the test will immediately fail (HALT) if run on a PDP-11/40 or PDP-11/45, because it tests the RESERVED
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instruction trap by using the MUL opcode, which was only reserved on early PDP-11's, like the PDP-11/20.
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After switching to a [PDP-11/20 configuration](/devices/pdp11/machine/1120/panel/debugger/), Test 14 uncovered a series of
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problems, including:
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- The DL11 transmitter must generate an interrupt *immediately* after being enabled
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- When a RESERVED instruction trap occurs with an invalid stack pointer, the RESERVED instruction trap must be immediately
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followed by a stack overflow trap
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- When a hardware interrupt occurs with an invalid stack pointer, the hardware interrupt trap must be immediately followed
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by a stack overflow trap (ie, before the first instruction of the hardware interrupt handler is executed)
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- When a BUS error occurs while fetching an opcode, the address of the opcode must be pushed by the trap handler
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- When a trap handler loads a new PSW that allows a hardware interrupt to be acknowledged, the acknowledgement must be delayed
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by one instruction
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And finally, the most interesting bug uncovered by Test 14 involved instructions like this:
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MOV R0,(R0)+
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Imagine that R0 contains 1000. PDPjs would write the value 1000 to address 1000 after auto-incrementing R0 to
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1002.
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Unfortunately, that's wrong -- for the PDP-11/20 anyway. Apparently, unlike later (micro-coded) models, the
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PDP-11/20 performs both source and destination address calculations *before* reading and writing the source and
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destination values. So, in the above example, the value 1002 must be written to address 1000.
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Don't confuse this behavior with the order in which the source and destination operands are processed (source operands
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are always decoded first, destination operands next), or with the fact that the auto-increment mode is actually a *post*-
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increment mode, whereas auto-decrement is a *pre*-decrement mode. Those things are also true, but they have nothing to do
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with this behavior.
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PDPjs resolved this by using a special (negative) value to indicate a register source operand, which is then converted
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to the register's current value after both operands have been decoded. Test 14 now passes.
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Next up: [Test 15](/apps/pdp11/tapes/diags/#test-15), which uncovered a couple more problems:
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- The DLL transmitter must *not* generate an interrupt if it is immediately *disabled* after being *enabled*
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- The (old) RTI instruction needs to manage the Trace flag like the (new) RTT instruction if the machine is a PDP-11/20
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Test 15 was written for the earliest PDP-11 models, which didn't have the RTT instruction, so when an RTI instruction
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enabled the Trace flag, action on the flag was deferred until after the next instruction. Newer models moved that deferred
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behavior to the RTT instruction, and changed RTI to act on the Trace flag immediately.
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This seems like a regrettable decision. It would have been far better for the new instruction to have the new behavior,
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and to leave the original RTI instruction alone, for maximum backward compatibility. DEC engineers probably regretted not
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having separate RTI and RTT instructions from the beginning, and they probably concluded that very little system software
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would be affected by changing RTI.
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Unfortunately, these diagnostics are one of the casualties of that decision, meaning that this diagnostic will *not* run
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successfully on an 11/70. And [Test 15](/apps/pdp11/tapes/diags/#test-15) is not alone in that regard.
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[Test 14](/apps/pdp11/tapes/diags/#test-14) also will not run properly on an 11/70; there again, the problem could have been
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avoided if DEC had decided to designate an opcode as *permanently* RESERVED, and then used that for all future RESERVED
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instruction tests, instead of using opcodes that would become new instructions later (eg, MUL).
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The most recent paper tape diagnostic I've run is the [11/70 CPU EXERCISER](/apps/pdp11/tapes/diags/#md-11-1170-cpu-exerciser),
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and after several more bug fixes, PDPjs passes that test now, too. That's something not even
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[SimH](https://github.com/simh/simh) is currently able to do (as of November 2016).
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---
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It's worth noting that all these diagnostic paper tape images are in the [Absolute Loader](/apps/pdp11/tapes/absloader/) format,
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but unlike tapes like [PDP-11 BASIC](/apps/pdp11/tapes/basic/), they don't include a start address, so you have to read the
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documentation for each test to learn the starting procedure, including any switches that should be set first.
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However, PDPjs makes life a bit simpler for you. As noted for other [DEC PDP-11 Tape Images](/apps/pdp11/tapes/), any
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"Absolute Format" tape can be read directly into RAM using the machine's **Read** button instead of **Load**, allowing you
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to bypass the usual multi-step process of loading the [Bootstrap Loader](/apps/pdp11/boot/bootstrap/) in order to load the
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[Absolute Loader](/apps/pdp11/tapes/absloader/) in order to load the desired tape.
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Moreover, PDPjs' JSON-encoded paper tape images support an *exec* property that allows the start address to be explicitly
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set, which will override any start address inside the image. For these diagnostics, I have set the start address (usually 200)
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via the *exec* property.
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*[@jeffpar](http://twitter.com/jeffpar)*
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*Nov 13, 2016*
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