IBM System/360

System/360 or short S/360 refers to a mainframe architecture of IBM from 1964. Previously, the IBM 700/7000 series offers. The System/360 was followed by the announcement in 1970 System/370.

The main architects were Frederick P. Brooks, Gerrit Blaauw, Gene Amdahl. The total line had Bob O. Evans. Participation was also, inter alia, Erich Bloch.

Design criteria

The main design criteria were:

  • General Purpose. Prior to the S/360 computers were optimized either for commercial or for scientific and technical purposes.
  • 8-bit character size. As opposed to the contemporary architecture, which worked with the 6-bit units, byte have been used for 8 bits.
  • 32 - or 64 -bit Gleitkommaworte with hexadecimal base.
  • Sign on integer values ​​is 1 bit
  • Packed decimal numbers stored can have variable lengths of 1 to 31 digits in some " high level languages ​​" such as RPG only 15 digits. Each digit occupies half a byte ( 4 bits binary coded decimal ), the sign in the last byte is the last 4 bits. "C" ( 1100) and "F" (1111 ) were the positive sign, "D" (1101 ), the negative sign. All other combinations were invalid sign. The position of the decimal point must be determined on the basis of the operands during programming.
  • Variable long strings have a length field and are not terminated with a special character.
  • EBCDIC instead of ASCII code because of the simpler implementation of the punched card code.
  • Waiver of a stack. This makes linkage conventions required, there must be secured for subprogram calls the status.
  • Generally indexed addressing, using a base register. Programs are in principle independent of physical addresses.
  • Binary addressing
  • All of the registers are general purpose registers which can be used both as accumulators and with the exception of the register 0 to the addressing. However, It is advisable to use only the registers from No. 3 ( up to max. 8) as the base register ( addressing). Registers 1 and 2 are of particular commands forcibly used (eg TRT). The registers 12 and 15 are used for calling subroutines and some are also needed for other purposes. A register is just enough for a memory range of 4096 bytes ( 4K bytes ) to address. In the machine instructions are for a memory address only 2 bytes are available, 4 bits for the register number and 12 bits for the displacement.
  • The general purpose registers are 32 bits wide, for addressing the right 24 bits are used, which allows an address space of 16 megabytes. The commands "BAL " and " BALR " (Branch and Link ), the return address is stored in a registry, but is saved in the left 4 bits of the condition code. Because of these features it is not so easy was possible with the successor systems to extend the address space above the 16MB out.

From version 67 also essential elements from the virtual memory were implemented after this had been previously removed from the draft early versions again. This was also found in the successor model System/370 again.

The S/360-Architektur has been continuously developed over the last forty years ( System/370, System/390 ) and culminates currently in the System z architecture.

Operating Systems

With the System/360 three operating systems appeared TOS/360 for installations without hard drives, DOS/360 for small and OS/360 for larger installations with discs. OS/360 is the forerunner of the current z / OS.

A peculiar feature, the system 360/20 dar. It was originally intended only as a punch card system (replacement of tabulating machines ) and had only a limited set of instructions ( machine instructions ). The register width was 16 bit with 8 instead of 16 registers.

Some of the operating systems for System/360 allowed multiprogramming: MFT ( multiprogramming with a fixed number of tasks) and MVT ( multiprogramming with a variable number of tasks). From MFT itself OS/VS1 developed in the System/370 and from MVT to MVS developed in System/370. With the Model 67 IBM also offered first timeshare systems.

Emulation

Besides the original System/360-Hardware the operating system OS/360 can now run on Windows and Linux systems with the free Hercules emulator.

Name Meaning

The system had claim to be all-encompassing. Therefore, we chose the number 360 as an indication of the maximum degree of an angle. However, the claim could not be fulfilled, so that the number has been defined differently in hindsight 3 was interpreted as IBM standard and 60 for a developed product in the 1960s. Therefore, the successor to the S/370 and S/390 were called.

Clones

In the Eastern Bloc three rows of computer systems have been developed. ESER Series I were largely identical with the IBM System/360. For this purpose, the assets included R40/EC 1040 (VEB Kombinat Robotron DDR) or EC 1022 ( EC EWM / Soviet Union).

z/OS Time-sharing Hercules (Emulator) IBM System/370 East Germany
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