Modular Computer Systems, Inc (MODCOMP) was a small minicomputer vendor that specialized in real-time applications. They were founded in 1970 in Fort Lauderdale, Florida. In the 1970s and 1980s, they produced a line of 16 and 32-bit mini-computers. Through the 1980s, MODCOMP lost market share as more powerful micro-computers became popular, and Digital Equipment Corporation's VAX and Alpha systems continued to grow. The company successfully survives today as a systems integrator.
Their first computer was the 16-bit MODCOMP III, introduced shortly after the company was founded. This had 15 general-purpose registers, and was initially offered with a 16-kilobyte (16,384 bytes), 18-mil magnetic core memory with an 800 ns cycle time, expandable to 128 kilobytes (131,072 bytes). The MODCOMP I followed for smaller applications, with only 3 general-purpose registers and a maximum of 64 kilobytes (65,536 bytes) of core. These machines were based on SSI and MSI TTL logic.
The MODCOMP II, introduced in 1972, maintained compatibility with the Modcomp III, while using some LSI circuits. The core architecture of the 16-bit machines included blocks of uncommitted opcodes and provisions for physical modularity that hint at the reasoning behind the company name. The MODCOMP IV was an upward compatible 32-bit machine with a paged memory management unit introduced in 1974. The minimum memory configuration was 32 kilobytes (32,768 bytes), expandable to 512 kilobytes (524,288 bytes), with access times of 500 to 800 nanoseconds (varying because of memory interleaving). The machine had 240 general purpose registers, addressable as 16 banks of 15 registers. The MMU contained 1024 address mapping registers, arranged as 4 page tables of 256 pages each (some of these page tables could be further subdivided if address spaces smaller than 128 kilobytes (131,072 bytes) were needed). Fields of the Program Status Doubleword were used to select the current active register bank and page table. The machine had a two-stage pipelined CPU, and a floating point unit. In many regards, the MODCOMP IV had potential as a competitor for the VAX, although the address space per process was limited to 64K 16-bit words; 256 pages of 256 words each, from the perspective of the MMU . Beginning in 1978, the MODCOMP IV was replaced by the MODCOMP Classic; the first Classic model was the 7810. This retained compatibility with the MODCOMP IV, while offering full support for 32-bit addressing. The later 9250 and 9260 continued to support both 16-bit and 32-bit applications.
Official NASA press release announcing MODCOMP partnership (page 84) [pdf]
Computing in Spaceflight - A detailed history of this era of NASA computers
Book by Marshall William McMurran, "NASA Control Computers" (chapter 6), section on MODCOMP
NASA Technical Reports of MODCOMP validation tests
Similar computers found in someone's garage
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NASA Mission Control Computer Chip
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