ND812

The 12-bit ND812, produced by Nuclear Data, Inc., was a commercial minicomputer developed for the scientific computing market. Nuclear Data introduced it in 1970 at a price under $10,000.

Description
The architecture has a simple programmed I/O bus, plus a DMA channel. The programmed I/O bus typically runs low to medium-speed peripherals, such as printers, teletypes, paper tape punches and readers, while DMA is used for cathode ray tube screens with a light pen, analog-to-digital converters, digital-to-analog converters, tape drives, disk drives.

The word size, 12 bits, is large enough to handle unsigned integers from 0 to 4095 – wide enough for controlling simple machinery. This is also enough to handle signed numbers from -2048 to +2047. This is higher precision than a slide rule or most analog computers. Twelve bits could also store two six-bit characters (note, six-bit isn't enough for two cases, unlike "fuller" ASCII character set). "ND Code" was one such 6-bit character encoding that included upper-case alphabetic, digit, a subset of punctuation and a few control characters. The ND812's basic configuration has a main memory of 4,096 twelve-bit words with a 2 microsecond cycle time. Memory is expandable to 16K words in 4K word increments. Bits within the word are numbered from most significant bit (bit 11) to least significant bit (bit 0).

The programming model consists of four accumulator registers: two main accumulators, J and K, and two sub accumulators, R and S. A rich set of arithmetic and logical operations are provided for the main accumulators and instructions are provided to exchange data between the main and sub accumulators. Conditional execution is provided through "skip" instructions. A condition is tested and the subsequent instruction is either executed or skipped depending on the result of the test. The subsequent instruction is usually a jump instruction when more than one instruction is needed for the case where the test fails.

Input/Output
The I/O facilities include programmable interrupts with 4-levels of priority that can trap to any location in the first 4K words of memory. I/O can transmit 12 or 24 bits, receive 12 or 24 bits, or transmit and receive 12 bits in a cycle. I/O instructions include 4 bits for creating pulses for peripheral control. I/O peripherals can be attached via 76 signal connector that allows for direct memory access by peripherals. DMA is accomplished by "cycle stealing" from the CPU to store words directly into the core memory system.

Nuclear Data provided interfaces to the following peripherals:
 * Bulk storage devices
 * Diablo Model 31 standard density disk cartridge
 * Diablo Model 31 high density disk cartridge
 * EDP fixed-head disk models 3008, 3016, 3032, 3064, or 3120
 * Magnetic tape I/O devices
 * Magnetic cassette tape
 * PEC 7-track magnetic tape
 * PEC 9-track magnetic tape
 * Paper tape I/O devices
 * Superior Electric Model TRP125-5 photoelectric tape reader (125 char/sec)
 * Dataterm Model HS300 photoelectric tape reader (300 char/sec)
 * Remex Model RPF1150B photoelectric tape reader (200 char/sec)
 * Remex Model RPF1075B mylar tape punch (75 char/sec)
 * Tally Model 1504 paper tape reader (60 char/sec)
 * Tally Model 1505 paper tape punch (60 char/sec)
 * Teletype Model BRPE11 paper tape punch (110 char/sec)
 * Hard copy I/O devices
 * Data Products Model 2410 line printer
 * Centronics Model 101 line printer (165 char/sec)
 * Franklin Model 1220 printer (20 lines/sec)
 * Franklin Model 1230 printer (30 lines/sec)
 * Hewlett-Packard Model 5050A printer (20 lines/sec)
 * Hewlett-Packard Model 5055A printer (10 lines/sec)
 * Calcomp digital incremental plotter

Programming facilities
The ND812 did not have an operating system, just a front panel and run and halt switches. The I/O facility allowed for peripherals to directly load programs into memory while the computer was halted and not executing instructions. Another option was to enter a short loader program that would be used to bootstrap the desired program from a peripheral such as a teletype or paper tape reader. Since core memory is non-volatile, shutting off the computer did not result in data or program loss.

A number of system programs were made available by Nuclear Data for use with the ND812: BASC-12 assembler, symbolic text editor, NUTRAN interpreter, and disk-based symbolic text editor,

BASC-12 Assembler
An assembler called BASC-12 was provided. BASC-12 was a two-pass assembler, with an optional third pass. Pass one generates a symbol table, pass two produces a binary output tape and pass three provides a listing of the program.

A sample of the assembler from the Principles of Programming the ND812 Computer manual is shown below:

/Input two unequal numbers "A" and "B", compare the two numbers /and determine which is larger, and output a literal statement /"A > B", or "B > A" as applicable. / /Input and store values for A & B                *200 Start,         TIF                     /Clear TTY flag JPS    Input           /Get value for A                 STJ     A                 JPS     Input           /Get value for B                 STJ     B / /Determine which of the two values is larger LDJ    A                 SBJ     B               /Subtract B from A                 SIP     J               /Test for A positive JMP    BRAN            /No! B > A                LDJ     ABCST           /Yes! A > B                SKIP                    /Skip next instruction BRAN,          LDJ     BACST / /Set up and output expression /                JPS     OUT STOP JMP    START / /Working or data storage area / A,             0                       /Constant A B,              0                       /Constant B ABCST,          AB                      /Address of A > B literal BACST,         BA                      /Address of B > A literal C260,          260                     /ASCII zone constant / /Input routine + ASCII zone strip / Input,         0                       /Entry point TIS JMP    .-1 TRF TCP                    /Echo input at teletype TOS JMP    .-1 SBJ    C260 JMP@   INPUT / /Output routine - Output ASCII expression / Out,           0                       /Entry point STJ    LOOP+1 LDJ    C5              /Set number of character constant STJ    CTR / /Output data loop / Loop,          TWLDJ 0                TCP TOS JMP    .-1 ISZ    LOOP+1 DSZ    CTR             /Test for all characters out JMP    LOOP            /No JMP@   Out             /Return C5,            5 CTR,           0 / /Output messages / AB,            215 212                301     /A 276    />                 302     /B BA,            215 212                302     /B 276    />                 301     /A $                                      /End character

NUTRAN
NUTRAN, a conversational, FORTRAN-like language, was provided. NUTRAN was intended for general scientific programming. A sample of NUTRAN is shown below:

1 PRINT 'INPUT VALUES FOR X AND Y' 2 INPUT X,Y 3 Z=X+Y 4 PRINT 'X+Y= ',Z 5 STOP

An example of the conversational nature of NUTRAN is shown below. is the command prompt and  is the input prompt.

>1.G INPUT VALUES FOR X AND Y :3 :2 X+Y=   .5000000E  1 >

Instruction formats
The instruction set consists of single and double word instructions. Operands can be immediate, direct or indirect. Immediate operands are encoded directly in the instruction as a literal value. Direct operands are encoded as the address of the operand. Indirect operands encode the address of the word containing a pointer to the operand.

Single word instructions
The displacement and sign bit allow single word instructions to address locations between -63 and +63 of the location of the instruction. Bit 4 of the instruction allows for a choice between indirect and direct addressing. When the displacement is used as an indirect address, the contents of the location which is +/-63 locations from the instruction location is used as a pointer to the actual operand.

Many single word instructions do not reference memory and use bits 4 and 5 as part of the specification of the operation.

Group 1 Format
Group 1 instructions perform arithmetic, logical, exchange and shifting functions on the accumulator registers. This includes hardware multiply and divide instructions. Bit 4 is set if the K register is affected. Bit 5 is set if the J register is affected. Both bits are set of both registers are affected.

Group 2 format
Group 2 format instructions test for internal conditions of the J and K accumulator registers, manipulate the overflow and flag status bits and provide complement, increment and negation operations on the J and K accumulator registers. Bits 9, 10 and 11 select the condition to be tested.

Two word instructions
Bit 9, Change Fields, inhibits the absolute address from referencing a different field than the one containing the instruction. When bit 8 is 1, the upper accumulator K is used with the instruction, otherwise the lower accumulator J is used. When bit 7 is 1, indirect addressing is used, otherwise direct addressing is used.

Status Word format
The status register doest not exist as a distinct register. It is the contents of several groups of indicators that are all stored in the J register when desired. The JPS and Int bits hold the current field contents that would be used during a JPS instruction or interrupt. The flag and overflow bits can be set explicitly from the J register contents with the RFOV instruction, but the other bits must be set by distinct instructions.

Subroutines
The ND812 processor provides a simple stack for operands, but doesn't use this mechanism for storing subroutine return addresses. Instead, the return address is stored in the target of the  instruction and then the register is updated to point to the location following the stored return address. To return from the subroutine, an indirect jump through the initial location of the subroutine restores the program counter to the instruction following the instruction.