Merge branch 'master' into gh-pages

_posts/2017-02-28-learning-the-pdp-10.md

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+---
+layout: post
+title: Learning the PDP-10
+date: 2017-02-28 22:00:00
+permalink: /blog/2017/02/28/
+---
+
+Since I've more or less achieved all my goals for the PCjs [PDP-11 Emulator](/devices/pdp11/machine/), I've decided
+to turn my attention to an even older historically important DEC computer: the [PDP-10](https://en.wikipedia.org/wiki/PDP-10#Cancellation_and_influence).  
+
+Having never used a real DEC PDP-10, I've had a steep learning curve creating a [PDP-10 Emulator](/modules/pdp10/).
+So far, the only [PDP-10 Hardware Documentation](/pubs/dec/pdp10/) I've read (or rather, skimmed) is the
+[PDP-10 System Reference Manual (1968)](http://archive.pcjs.org/pubs/dec/pdp10/ka10/DEC-10-HGAA-D_PDP-10_System_Reference_Manual_May1968.pdf).
+
+To make the process a little less painful, most of my initial development efforts have centered on the built-in PDP-10
+Debugger.  In addition to the usual disassembler capabilities, I've also included an assembler, so that I can easily enter
+instruction sequences like this:
+
+	a 100;
+	hrli 1,111111;
+	hrri 1,444444;
+	hlr 2,1;
+	hrl 2,1;
+	end
+
+and immediately see the corresponding opcodes:
+
+	>> u 100 l4
+	000100: 505040 111111  HRLI    1,111111
+	000101: 541040 444444  HRRI    1,444444
+	000102: 544100 000001  HLR     2,1
+	000103: 504100 000001  HRL     2,1
+
+It's not a full-featured assembler; it's little more than a brute-force reverse look-up through the disassembler's
+opcode-to-mnemonic tables.  But it gets the job done.
+
+Next, I've written an initial set of [PDP-10 Opcode Tests](/apps/pdp10/tests/) and compared the results to those produced
+by [SIMH](https://github.com/simh/simh), and so far, so good.  More than anything else, that's just a testament to DEC's
+excellent documentation.
+
+One of the challenges in creating a JavaScript-based [PDP-10 Emulator](/modules/pdp10/) involves 36-bit arithmetic, because
+JavaScript bit-wise operators maintain only 32 bits.  Extreme care must be taken to use only *arithmetic* operators whenever
+accessing and/or preserving the 4 most-significant bits in any 36-bit value.
+
+So, before I started this project, I wrote a proof-of-concept [Int36 Class](/modules/shared/lib/int36.js) that supports 36-bit
+addition, subtraction, multiplication, and division, as well as a number of helper functions (e.g., *toString()*).  Further
+adaptation will be required, however, because **Int36** multiplication produces 72-bit results and **Int36** division uses 72-bit
+dividends, whereas the PDP-10 uses "double length fixed point" values that contain only 70 bits of magnitude with two matching
+sign bits.
+
+I just finished writing opcode handlers for all the PDP-10 "Half Word" instructions (i.e., *HLL*, *HRL*, *HRR*, *HLR*, with all
+mode variations and bit extension options), but that's just the tip of the iceberg.  Hopefully within another month or
+so I'll be able to find the time to finish the rest of the PDP-10 opcode handlers.
+
+And then maybe we'll be able to run some *real* PDP-10 software in our browsers.
+
+*[@jeffpar](http://twitter.com/jeffpar)*  
+*Feb 28, 2017*

pubs/dec/pdp10/README.md

@@ -5,7 +5,7 @@ permalink: /pubs/dec/pdp10/
 ---
 
 PDP-10 Hardware Publications
----
+----------------------------
 
 This page summarizes PDP-10 documentation that was useful in creating [PDPjs](/docs/pdpjs/).  In most cases,
 these documents came from the PDF Document Archive at [bitsavers.org](http://bitsavers.org),  in the [/pdf/dec/pdp10/](http://bitsavers.org/pdf/dec/pdp10/)