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 being 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 the RAM
starting at address 0 after which is cedes control to the CPU. The
instruction and return pointers are initialised as 0 and the first
address after RAM respectively, while other registers and RAM are
uninitialised.

Emulator
--------

The emulator runs at roughly 500 KIPS, has the full 65520 bytes of RAM,
and interacts with memory mapped devices at roughly 1000 CPS. The speed
limitations can be removed by compiling the emulator with the argument
-dfast.

Input and output are handled by an emulated glass teletype terminal with
local echo. Note that of the control characters only bell, backspace,
line feed, and carriage return are used by the terminal, and the
backspace and delete keys are tied to their respective characters and
non-character keys to 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.