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.

Speed
-----

Thingamajig does not have a prescribed speed. The emulator runs at
roughly 500 KIPS.

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 addresses FFF0-FFFF are
reserved for memory mapped devices; the instruction and return pointers
cannot have values higher than FFEF and FFF0 respectively to avoid them.
The instruction and return pointers are initialised as 0 and FFF0
respectively, while other registers and memory are unitialised.

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 being optional. Note that the assembler does not check for
addresses or references to reserved addresses.

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.

Memory-Mapped Devices
---------------------

Input and output are mapped to address FFFF; the emulator emulates a
roughly 1000 CPS glass teletype with local echo.

Arbitrary devices can be mapped to the other reserved addresses.

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. If you wish to use a different setup you have
to modify the code yourself.

Initial Program Loader
----------------------

At boot the initial program loader (IPL) loads a program to the memory
starting from address 0 after which is cedes control to the CPU. The
emulator loads the program from a ROM file.