added readme, license

LICENSE.txt

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+Copyright 2023 Keelan Lightfoot
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the “Software”), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# disasm4000sx
+
+A disassembler for the stack based virtual machine running on a circa 1987 Trimble 4000SX GPS receiver.
+
+The VM seems to have a lot in common with the UCSD Pascal P-Code virtual machine, with similar classes of instructions, but not similar opcodes. It uses Pascal style strings, which leads me to believe this might have been a commercially sold Pascal implementation.
+
+I haven't figured out some opcodes yet, but I've figured out enough to determine how many extension bytes each opcode will have so that they don't mess up the disassembly.
+
+More information can be found here:
+
+http://beefchicken.com/gps/trimble/4000/sxdeepdive
+
+
+# Building
+
+This project builds under Go 1.20. Install go, enter the directory of the project, then run `go install`
+
+# Usage
+
+The disassembler reads from binary ROM dumps. A ROM dump consists of a directory full of ROM files, and a `map.json` file that instructs the disassembler on how to reassembled the roms. For example:
+
+    {
+        "roms": [
+            {
+                "file": "U36.rom",
+                "start": "0x000000", "end":   "0x003FFE",
+                "version": "3.30",
+                "device": "27C64",
+                "checksum": "0x52EA"
+            },
+            {
+                "file": "U44.rom",
+                "start": "0x000001", "end": "0x003FFF",
+                "version": "3.31",
+                "device": "27C64",
+                "checksum": "0xD16B"
+            },
+            {
+                "file": "U37.rom",
+                "start": "0x090000", "end": "0x09FFFE",
+                "version": "3.30",
+                "device": "AT27C256R",
+                "checksum": "0x2500"
+            },
+            {
+                "file": "U45.rom",
+                "start": "0x090001", "end": "0x09FFFF",
+                "version": "3.30",
+                "device": "AT27C256R",
+                "checksum": "0x8E92"
+            },
+            {
+                "file": "U38.rom",
+                "start": "0x0A0000", "end": "0x0AFFFE",
+                "version": "3.30",
+                "device": "AT27C256R",
+                "checksum": "0xE240"
+            },
+            {
+                "file": "U46.rom",
+                "start": "0x0A0001", "end": "0x0AFFFF",
+                "version": "3.30",
+                "device": "AT27C256R",
+                "checksum": "0x0717"
+            }
+        ],
+        "labels": [
+            {"address": "0x0034AA", "label": "CLEARSCREEN"},
+            {"address": "0x095BD8", "label": "SLEEP1SECOND"},
+            {"address": "0x0034C8", "label": "SETSEGMENTS"}
+        ]
+    }
+
+
+The disassembler will automatically figure out which ROMs are pairs.
+
+To run the disassembler:
+
+`disasm4000sx <path to rom directory>`
+
+for example
+
+`disasm4000sx 331`
+
+# License
+
+Distributed under the MIT License. See LICENSE.txt for more information.
+
+# Contact
+
+Keelan Lightfoot - http://beefchicken.com/contact/
+
+Project Link: http://code.beefchicken.com/keelan/disasm4000sx
+