4.1 KiB
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, 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 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 address argument are 8-bit and those with 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, ADDR RX = *ADDR B STORE RX, ADDR *ADDR = RX Written as "STORE ADDR, RX" in assembly for the sake of consistency.
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 (when read from) and output (when written to) are mapped to address FFFF. The emulator emulates a dumb terminal with local echo.
Arbitrary devices can be mapped to the other reserved addresses.
In Linux the emulator can be compiled with support for a line 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 settape program. If you wish to use a different setup you have to modify the code yourself.
Initial Program Loader
At boot the initial program loader 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 file.