During the late 1960s, Interdata Corporation introduced a line of compatible computers under the imaginative name of "Model x", where "x" was a single number. This is a Model 4.
The machine sports a 16-bit architecture with a maximum address space of 64 kB. The memory is byte-addressed, although it is word-accessed, and was installed in 4 kB chunks. This example has 8 kB of core memory installed. There are 16 general purpose registers, 15 of which can be used as index registers. The program counter is 16 bits wide.
Like most minicomputer architectures of the time, the Model 4 has separate memory and I/O busses and unique instructions to access each bus. The instruction set, from a user perspective, looks almost exactly like the IBM Basic Assembly Language (BAL) which came into use with the system 360. It has Register- to- Register, Register- to- Storage, and Register- to- indeXed storage class instructions. RR instructions are 16 bits long; RS and RX instructions are 32 bits long.
The machine is implemented in DTL (Diode- Transistor- Logic) and is
microcoded. The microcode resides in pulse- transformer ROM, which is a cousin
of the "core-rope" memory used in the Apollo Guidance Computers. Physically,
most of the boards are of wire-wrap construction, although the four bit-
slice modules (containing registers and ALU) are soldered construction. The
wire- wrap boards are unique in that each is a "motherboard" onto which
mount many individually replacable "daughter- boards" each of which contains
only a few DTL ICs.
To the right is an image of the CPU card- cage of the Model 4 with various subsystems highlighted in different colours. The memory systems appear in the purple areas, CPU control in green, basic I/O (console and TTY) in blue, and microcode ROM in yellow. The bit- sliced ALU is in red.
The other boards are custom cards and control A/D, D/A, and timing functions to facilitate the use of the computer as a laboratory instrument. This machine was used in psychology experiments during the late '60s and '70s. This particular machine also has a high- speed paper tape reader attached to it.
The microword in this machine is 16 bits wide and the microinstruction set is remarkably similar to the user- level instruction sets available on other computers of the time. The fun bit with the microcode, though, is that the microcode must do everything in the machine, including controlling core memory access.
The primary differences, at a user level between the Model "x" lines were the number of instructions included in the basic repertoire. The Model 2 and Model 3 sported subsets of the Model 4 set. A single instruction- set reference manual was published for all the different machines. There were striking differences at the micromachine level, though. For instance the Model 3 only had an eight bit ALU where the Model 4 has a full- width sixteen bitter.
Incredibly, there appear to be sites that still use the Model 4 as I've
seen service companies with presences on the Internet that claim to still
support the machine! Given that DTL logic hasn't been manufactured in
several years, the longevity of these machines is stunning.