Thingamajig/readme.md

Thingamajig
===========

Thingamajig is a RISC-y and MISC-y homebrew computer architecture. Its
git repository can be found at
https://ahti.space/git/crazyettin/Thingamajig.

Included Software
-----------------

The repository includes an emulator implementation of Thingamajig with a
control program for the emulated punched tape reader and punch, and an
assembler and a disassembler, all written in FreePascal. It also
includes couple of simple example programs for Thingamajig written in
assembly.

Registers and Memory
--------------------

  * 24-bit instruction register IR
  * 16-bit instruction and return pointers IP and RP
  * 8-bit general-purpose registers R0-R3
  * 8-bit memory locations 0-FFFF

Multi-byte values are big-endian. Memory locations 0-FFEF are used for
RAM while FFF0-FFFF are reserved for memory mapped devices. Input and
output are mapped to address FFFF, while arbitrary devices can be mapped
to the other reserved addresses.

Instructions
------------

Instructions without an immediate or address argument are 8-bit, those
with an immediate one 16-bit, and those with an address one 24-bit. The
instruction pointer is incremented before being accessed or modified.

0   HALT
1   RET                    IP = *RP; RP += 2

2   SHL     RX             RX <<= 1    Logical shifts
3   SHR     RX             RX >>= 1
4   ROL     RX             RX <<= 1    Rotating shifts
5   ROR     RX             RX >>= 1

6   NAND    RX, RY         RX = ~(RX & RY)
7   AND     RX, RY         RX &= RY
8   OR      RX, RY         RX |= RY
9   XOR     RX, RY         RX ^= RY

A   LOAD    RX, ~0, IMM    RX = IMM      Written as "LOAD RX, #IMM"
                 0, ADDR   RX = *ADDR    Written as "LOAD RX, ADDR"
B   STORE       RY, ADDR   *ADDR = RY    Written as "STORE ADDR, RY"

C   BREQ    RX, RY, ADDR   if (RX == RY) IP = ADDR
D   BRNEQ   RX, RY, ADDR   if (RX != RY) IP = ADDR
E   CLEQ    RX, RY, ADDR   if (RX == RY) {RP -= 2; *RP = IP; IP = ADDR}
F   CLNEQ   RX, RY, ADDR   if (RX != RY) {RP -= 2; *RP = IP; IP = ADDR}

Assembly Language
-----------------

Lines of assembly are of the following form:

LABEL: OPER ARG1, ARG2, ARG3 ;Comment

The language is case-insensitive and uses hexadecimal numbers. A label
can consist of any alphanumeric characters as long as it is not
interpretable as a hexadecimal number. The label, instruction, and
comment elements are all optional, as is spacing between the arguments.
For the arguments of each instruction see the previous section.

Address arguments can be either absolute addresses or references to or
relative to a label. Relative references are of the form LABEL +/- N;
the spacing is optional.

In addition to the true instructions there are three
pseudo-instructions. ORG defines the starting address of the program: it
can only occur as the first instruction and cannot have a label, and is
not required if the starting address is 0. DATA introduces a byte of
data. ADDR introduces two bytes of data containing the address of a
reference to or relative to a label.

Boot
----

At boot the initial program loader (IPL) loads a program to RAM starting
at address 0 after which is cedes control to the CPU. If an
implementation has a front panel the IPL is optional. The instruction
and return pointers are initialised as 0 and the first address after RAM
respectively, while other registers and RAM are uninitialised.

Emulator
--------

By default the emulator runs at roughly 500 KIPS, has 2 KiB of RAM, and
interacts with memory mapped devices at roughly 1000 B/s. The arguments
-dRAM4, -dRAM8, -dRAM16, -dRAM32, and -dRAM64 can be used to compile
the emulator with 4, 8, 16, 32, or 64 KiB (minus the reserved addresses)
of RAM respectively instead and the speed limitations can be removed
with the argument -dfast.

Input and output are handled by an emulated glass teletype terminal with
local echo on by default. Of the control characters bell, backspace,
line feed, carriage return, and device control characters two and four
are used by the terminal: the device control characters are used to turn
the local echo on or off respectively, while the rest have their
standard uses. The backspace and delete keys input their respective
characters and non-character keys null.

In Linux the emulator can be compiled with support for a character
printer and an emulated punched tape reader and punch with the arguments
-dprinter and -dtape respectively. The printer is mapped to address FFFE
and the tape reader and punch to FFFD. The printer prints into
/dev/usb/lp0 and the tape files read from and punched to are (re)set
using the program tapectl.

The IPL loads a program from a file specified when launching the
emulator.