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Thingamajig v2.0

Thingamajig v2.0 is a RISC/MISC homebrew computer architecture. Its git repository can be found at

Included Software

The repository includes an emulator implementation of Thingamajig with device control programs, 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. When interacting with memory mapped devices Thingamajig will stop processing to wait for the device to be ready if needed.


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, N RX <<= N (logical) Shifts of 1-4 steps, 3 SHR RX, N RX >>= N (logical) with 4 encoded as 0 4 ROL RX, N RX <<= N (rotating) in machine code. 5 ROR RX, N RX >>= N (rotating)

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:


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 sets the location of the following code and data; as it has no direct equivalent in machine code it cannot have a label. The default starting address of 0 does not need to be indicated with ORG. DATA introduces a byte of data. ADDR introduces two bytes of data containing the address of a reference to or relative to a label.


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 and Device Control Programs


  • emulator (-v) program (2> verbose_output)
  • tapectl (-r tape) (-p tape)
  • floppyctl (-0 disc) (-1 disc)
  • modemctl -o/[-a address:port]/[-d address:port]/-h

By default the emulator runs at roughly 500 KIPS and has 2 KiB of RAM. 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. When run with the argument -v the current state of the registers is output to stderr before each instruction.

Input and output are handled by an emulated roughly 1000 CPS ASCII-compatible glass teletype terminal with local echo on by default. Of the control characters bell (^G), backspace (^H), line feed (^J), carriage return (^M), and device control characters two (^R) and four (^T) are used by the terminal: the device control characters are used to turn the local echo on and 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, an emulated high speed 8-bit paper tape reader and punch, an emulated two-drive 8" floppy disc system, a modem, and an input status register with the arguments -dprinter, -dtape, -dfloppy, -dmodem, and -dstatus respectively. Full 64 KiB of RAM and all of these options can also be enabled with the argument -dfull. The printer is mapped to address FFFE, the tape reader and punch to FFFD, the disc system to FFFB and FFFC, the modem to FFF9 and FFFA, and the input status register to FFF8.

The printer prints into /dev/usb/lp0. The tape files read from and punched to are (re)set using the program tapectl with the arguments -r and -p respectively and the disc files in drives 0 and 1 using the program floppyctl with the arguments -0 and -1 respectively. The disc system uses hard sectored single-sided discs with 77 tracks of 32 sectors of 137 bytes, or 337568 bytes in total: the disc files must be of this size. The modem is controlled by the program modemctl: the options -o and -a are used to set the originate and answer modes, the latter with the address and port to be listened; -d to dial an address and port to be called; and -h to hang. Dialing when in answer mode automatically switches the modem to originate mode. The modem hangs automatically if the connection is lost or hanged from the other side, if another address and port are dialed, if a mode change command is given, or if the emulator is halted.

The floppy disc system uses two ports: command at FFFB and data at FFFC. Only the low nibble of the command port is used, consisting of a three-bit command followed by a one-bit drive number used as an argument by instructions 1 and 4-7. The track or sector to be accessed is input to and the buffer accessed sequentially through the data port after the relevant command is input to the command port. New commands other than resetting the system are ignored until the current command is fully executed. Note that each sector begins with a synchronisation bit that must always be set.

The commands for the disc system are: 0: Reset the system. 1: Format a disc. 2: Read a sector from the buffer to the computer. 3: Write a sector from the computer to the buffer. 4: Set the track to be accessed. 5: Set the sector to be accessed. 6: Read a sector from a disc to the buffer. 7: Write a sector from the buffer to a disc.

The modem also uses two ports: status at FFF9 and data at FFFA. Only the least significant bit of the status port is used, indicating whether the modem is connected or not. Unsetting the bit hangs the modem while setting it in answer mode hangs any current call and listens for an incoming call to answer. Setting the bit in originate mode is ignored.

The input status register can be used to check if a device is ready to be read from. The reserved addresses FFF8-FFFF are mapped to the bits 7-0: a set bit means either that the device is ready or that no input device is mapped to the address in question.

The IPL loads the program specified as an argument when running the emulator.

Assembler and Disassembler


  • assembler program (< input)
  • disassembler program (> output)

Both the assembler and the disassembler are run with a program as their sole argument: they take their input from and print their output to stdin and stdout respectively.

An initial gap created with ORG is not included in an assembled program. All possible interpretations of a disassembled program are included in its listing, resulting in overlapping information.



  • Added the ability to shift a register multiple steps in one instruction
  • Changed the register argument of STORE from RX to RY
  • Added support for an emulated floppy disc system and modem and an input status register to the emulator
  • Added the ability to use ORG non-initially to the assembler
  • Changed tapectl so that it uses arguments for input


  • Initial release