LINC: New emulator for the classic LINC.

This emulates the classic LINC.  The design was settled in 1965,
increasing memory to 2048 words, and adding a Z register, an overflow
flag, and an interrupt facility.

linc/README.md

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+# LINC emulator
+
+This is an emulator for the classic LINC from 1965.
+
+### Devices
+
+- CPU - LINC processor.  If throttled to 125k, approximately original speed.
+- CRT - CRT display.  It can be disabled to run headless.
+- DPY - point-plotting display controller.
+- KBD - keyboard.  Input comes from typing in the CRT window.
+- SAM - sampled analog inputs.
+- TAPE - four tape drives.
+- TTY - teletype.
+
+### LOAD
+
+Software can be loaded into memory using the `LOAD` command.  It will
+normally read a file with 16-bit little endian words; the most
+significant four bits of each word should be zero.  If the `-O` switch
+is supplied, the input is a text file with octal numbers separated by
+whitespace.
+
+To start reading from some particular position in the input file, add
+`OFFSET=` after the file name and supply a number in octal.
+
+To specify where in the memory to put the data, add `START=` after the
+file name; the default is 0.  To specify how many words to load, use
+`LENGTH=`; the default is to write until the end of memory.
+
+A binary input file can be a tape image.  A plain image is 512 blocks
+of 256 words each, totalling 262144 bytes.  If the `-E` switch is
+used, there is a check for the extended image format that can have
+empty guard blocks at the beginning and the end.  The last three words
+in the extended format specify the block size, first forward block
+number, and first reverse block number.
+
+To state which tape block to start reading, add `BLOCK=` after the
+file name and supply an octal number in the range 0-777.
+
+### DO
+
+The SIMH `DO` command has been modified.  With arguments, it will
+execute a script file as normal.  Without argument, it acts like the
+DO button on the LINC control panel.  This executes one instruction
+from the left switches, with the right switches providing a second
+word if needed.
+
+### Tape drives
+
+To mount a tape image *file* on drive *n*, type `ATTACH TAPE<n>
+<file>`.  The plain or extended image format will be detected
+automatically.  Tape drives are numbered 0, 1, 4, and 5.
+
+The `BOOT TAPEn` command will act like entering a tape read command in
+the switches and starting the computer.  The default is to read eight
+blocks starting at 300, and start from location 20.  This is the
+conventional way to run LAP6.  You can also add `RDC=` or `RCG=` to
+boot some particular blocks.  `START=` can be used to specify the
+start address; it defaults to 20.
+
+### Keyboard
+
+The keys `0-9`, `A-Z`, and `Space` are mapped to their corresponding
+LINC keys.  `Enter` is mapped to `EOL`, `Delete` and `Backspace` are
+mapped to `DEL`, and `Shift` is mapped to `CASE`.  To type an upper
+case symbol on some key, press `CASE`, release it, and then type the
+key.  For convenience, `Alt` is mapped to `META`.
+
+The remaining keys are mapped thusly:
+- `F1` is mapped to the `pu` key.
+- `=` is mapped to the `i=` key.
+- `-` and `,` are mapped to the `-,` key.
+- `.` is mapped to the `+. key.
+- `\` is mapped to the `|⊟` key.
+- `[` and `left backslash` are mapped to the `*[` key.
+
+The remaining upper case symbols:
+- `CASE A` - `"`.
+- `CASE B` - `„`.
+- `CASE C` - `<`.
+- `CASE D` - `>`.
+- `CASE E` - `]`.
+- `CASE F` - `*`.
+- `CASE G` - `:`.
+- `CASE Space` - `?`.
+
+### Teletype
+
+The TTY device implmenents a teletype for printing output.  When a
+file is attached, it will receive text decoded at 110 baud from relay
+output 0.
+
+Some characters are translated by LAP6 from the LINC character to ASCII:
+- `i` to `&`
+- `p` to `'`
+- `|` to `\`
+- `u` to `%`
+- `⊟` to `$`
+- `_` to `@`
+- `"` to `^`
+- `„` to `;`
+
+### CPU
+
+Registers:
+- P - Instruction location, 10 bits.
+- C - Current instruction, 12 bits.
+- S - Memory address, 12 bits - 11 for address, 1 for halfword select.
+- B - Memory data buffer, 12 bits.
+- A - Accumulator, 12 bits - one's complement.
+- Z - Various, 12 bits.
+- L - Link, 1 bit.
+- OVF - Overflow, 1 bit.
+- IBZ - Tape interblock zone, 1 bit.
+- ENI - Interrupt enabled, 1 bit.
+- PINFF - Pause Interrupt enabled, 1 bit.
+
+Switches:
+- LSW - Left switches, 12 bits.
+- RSW - Right switches, 12 bits.
+- SSW - Sense switches, 6 bits.
+
+Inputs:
+- INTREQ, Interrupt request, 1 bit.
+- R - Relays, 6 bits.
+- XL - External levels, 12 of 1 bit each.
+- SAM - Sampled analog inputs, 16 of 8 bits each; one's complement.
+
+### Documentation
+
+Programming:
+https://bitsavers.org/pdf/washingtonUniversity/linc/Programming_the_LINC_Second_Edition_Jan69.pdf
+
+Using the LAP6 operating system, text editor, assembler, tape filing system:
+https://bitsavers.org/pdf/washingtonUniversity/linc/LINC_Reference_Manuals/LINC_Vol_16_Section_3_LAP6_Handbook_May67.pdf