【Mitsubishi Electric】 MELCOM 7000 Series

Mitsubishi Electric produced the MELCOM 7000 series through a technology-sharing agreement with the American firm XDS, in order to respond to the growing demand for mainframe computers. The series was announced in 1970. The MELCOM 7000 series consisted of the MELCOM 7500 and MELCOM 7700 models. One of the biggest distinction of the MELCOM 7000 series was its simultaneous support for four processing types: namely, batch processing, time-sharing processing, real-time processing, and remote batch processing. It was one of a handful of computers in the world, particularly in time-sharing processing, with sufficiently advanced functionality to allow up to 128 users to access and use the computer simultaneously. This functionality included dynamic relocation of programs and high-speed fixed-head magnetic disk units for swapping programs with data transfer speed of 3,000 kilobytes per second.

The system consisted of a main memory of up to 512 kilobytes in its center, and a central processing unit (CPU) and up to eight I/O processing devices connected to the main memory via memory buses.

The main memory consisted of up to eight memory modules, each of which could operate independently. Because of this independence, interleaving was used to assign addresses so the computer could access different memory modules when continuous addresses were accessed. The ability to overlap accesses reduced the effective main-memory access times and, thereby, boosted the computer’s processing speed. Addresses could be assigned using two-way interleaving and four-way interleaving. The main memory’s cycle time was 0.85 microseconds, but with interleaving, the main memory operated with an effective cycle time of just 0.56 microseconds.

The MELCOM 7000 series had 16 general-purpose registers for calculations, seven of which were used as index registers. The MELCOM 7500 could be expanded to 16 pairs of general-purpose registers and the MELCOM 7700 to 32 pairs of general-purpose registers. When multiprogramming, programs could be switched at high speed by allocating different registers to each program.

Addresses within instructions were word addresses. The index value indicated the displacement with the size of the instruction’s operand (byte, half word, word, or double word). As Figure 1 shows, the operand was converted to a displacement in bytes by shifting the index register’s value left by one bit if a half word, two bits if a word, and three bits if a double word. This was added to the 19-bit byte address which added two zero bits after the address in the instruction.

Figure 1: Auto address adjustment

Figure 1: Auto address adjustment

Figure 2 illustrates the operation of the dynamic program relocation function. This function divides logical addresses into an 8-bit page number and an 11-bit byte address within the page. It then looks up the physical page address corresponding to the page number in the address-mapping register, which contains 256 physical page address entries. This physical page address is concatenated with the byte address within the page to create the final physical address. With this function, each page is distributed in main memory, enabling the main memory to be used more effectively. The computer also had a 256-entry access-protection register with two-bit access control code for each page number. During address conversions, the corresponding access control code was looked up to check whether an instruction was permitted to access a particular address.

Figure 2: Dynamic program relocation

Figure 2: Dynamic program relocation

The MELCOM 7000 series had a multiplexer I/O processor, which connected with multiple low-to-medium-speed peripherals, and a selector I/O processor, which connected with high-speed peripherals. Together, the computer accepted up to eight I/O processors.

Users could select the operating system from RBM, BPM, BTM, and UTM.

The main specifications of the MELCOM 7000 series machines are given below.

Main memory Capacity Max. 512 KB
Memory modules 16 KB to 64 KB (max. eight memory modules)
Word length 32bits
Access length One word
Cycle time 0.85 μs (effective cycle time of 0.56 μs when interleaving is used)
CPU Data formats Fixed-point binary numbers (half word, word, double word)
Floating-point binary numbers (single precision, double precision)
Fixed-point decimal numbers (variable length)
Logical numbers (byte, word)
Characters (variable length)
Instruction length One word
Addressing Direct, indirect, indexed, indirect indexed
Computation speeds   MELCOM 7500 MELCOM 7700
Addition of one-word fixed-point binary numbers 2.0μs 1.4μs
Addition of single-precision floating-point numbers 4.9μs 3.3μs
Addition of double-precision floating-point numbers 9.9μs 4.1μs
Branching 1.2μs 1.0μs
Addition of 5-digit decimal numbers - 20.3μs
Data transfer (eight bytes) - 10.6μs
File memory units   Storage size Transfer speed Average access time
High-speed magnetic disk unit 5.4MB 3,000KB/s 17ms
6.2MB 384KB/s 17ms
0.75 to 3MB 188KB/s 17ms
Magnetic disk pack unit 24.5MB 312KB/s 87.5ms
49MB 312KB/s 87.5ms
Magnetic card cartridge 226MB 83KB/s 125ms
I/O devices Magnetic tape unit
Card reader
Line printer
Graphical I/O devices Plotter
Graphic display

MELCOM 7000 Series