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_posts/2016-10-21-booting-pdp-11-basic.md

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+---
+layout: post
+title: Booting PDP-11 BASIC
+date: 2016-10-21 23:00:00
+permalink: /blog/2016/10/21/
+machines:
+  - id: test1120
+    type: pdp11
+    debugger: true
+    autoMount: ''
+    config: /devices/pdp11/machine/1120/basic/debugger/machine.xml
+---
+
+[PDPjs](http://www.pcjs.org/devices/pdp11/machine/) can now simulate a PDP-11/20.  It was one of the first PDP-11
+models, and since it had no MMU, it was limited to a maximum of 56Kb of RAM (or as DEC would say, 28K words), since
+the top 8Kb (or 4K words) of its 16-bit address space was reserved for UNIBUS devices.
+
+The first PDPjs test of a PDP-11/20 configuration was attempting to run [PDP-11 BASIC](/apps/pdp11/tapes/basic/), by loading
+it directly into memory from the original paper tape image.  This required changes to the [RAM](/modules/pdp11/lib/ram.js)
+component, including a new *loadImage()* interface that understands DEC's paper tape format.
+
+To provide a more "authentic" experience, there is also a new [PC11 Paper Tape](/modules/pdp11/lib/pc11.js) component,
+and machines can now be configured to include a virtual paper tape reader, with an [assortment of paper tapes](/devices/pdp11/pc11/)
+ready to be attached.
+
+In the real world, before you could load *any* paper tape, you had to first enter a
+[Bootstrap Loader](/apps/pdp11/boot/bootstrap/).  PDPjs has simplified that process by including the Bootstrap Loader as
+just another paper tape image that you can "Load" directly into memory.  Once that has been loaded, you can then read any
+other paper tape image, once you "Attach" it to the paper tape reader.
+
+{% include machine.html id="test1120" %}
+
+*[@jeffpar](http://twitter.com/jeffpar)*  
+*Oct 21, 2016*