IBM System/360 Model 30
| Company type | Computer model |
|---|---|
| Industry | Computing / Mainframe computers |
| Founded | April 1964 |
| Defunct | October 1977 |
| Headquarters | Endicott , United States |
| Products | Mainframe computer |
| Parent | IBM |
The IBM System/360 Model 30 served as the entry-level option within IBM's System/360 series, announced in April 1964, released in 1965, and discontinued by October 1977. Developed by IBM's General Systems Division located in Endicott, New York, production took place both at this facility and various other IBM manufacturing sites worldwide.
History
As an essential component of the IBM mainframe range, the Model 30 was introduced as the least powerful System/360 model that maintained full compatibility. In contrast to the lower-tier IBM System/360 Model 20, which offered limited instruction set compatibility, the Model 30 supported the entire System/360 instruction repertoire. This capability allowed it to operate independently, serve as a communication node, or function as a satellite processor within more extensive system configurations.
The first unit was delivered in June 1965 to McDonnell Aircraft. Alongside the Model 40, the Model 30 became one of the highest revenue contributors in the System/360 lineup, together representing more than half of all units sold. This commercial success highlighted its critical role in IBM’s approach to delivering scalable computing solutions tailored to various business requirements.
Models
Initially, four variants of the 360/30 were available, distinguished by their core memory sizes: C30 with 8K, D30 with 16K, E30 with 32K, and F30 with 64K.[1]
Two distinct versions existed: the original 30-1 and the subsequent 30-2. The 30-1 operated with a storage cycle time of 2.0 microseconds but was superseded after about 1,000 units by the faster 1.5-microsecond cycle time 30-2. The latter also featured a different front panel design, where indicator lights were positioned behind a stencil rather than being exposed. Additionally, the 30-2 introduced a DC30 model equipped with 24K of memory.[1]
The IBM System/360 Basic Operating System Programmer's Guide from September 1967 acknowledged support for this intermediate memory size of 24K on the Model 30.[2]
96K upgrade
In response to competition offering systems with up to 128K memory, IBM launched an upgrade for the 360/30 that expanded its memory capacity to 96K. This enhancement considerably boosted performance by allowing tape and disk sorting operations to be completed in minutes instead of hours.[3] The design of the system’s front panel implies that support for memory beyond 64K had been contemplated during its development phase.[4]
Microcode
The Model 30 CPU utilized an 8-bit microarchitecture featuring a limited number of hardware registers; all functions visible to programmers were implemented through microprogram emulation. Processing a single four-byte word required a minimum of six microseconds, based on a storage access cycle time of 1.5 microseconds.[5]
Its microcode was housed in CCROS (Card Capacitor Read-Only Storage), a technology developed at Endicott. Originally, both the Model 30 and Model 40 were intended to use transformer read-only storage (TROS) from IBM Hursley, but CCROS proved more economical. This system employed Mylar cards matching standard IBM punched-card dimensions, enabling microcode updates through a keypunch. Each card contained 720 bits; altogether, the microcode consisted of 4,032 words of 60 bits each. The Mylar cards incorporated copper tabs and access lines; punched holes removed tabs to represent zeros while unpunched areas were interpreted as ones.[6]
System configuration
| Standard early Model 30 system setup:[7] | |
|---|---|
| Model 30 processor | IBM 2030 Central Processing Unit *32 KB memory *storage protection capability *core instruction set *decimal arithmetic instructions[NB 1] *one multiplexor channel *one selector channel *interval timer |
| Operator console | IBM 1052 Typewriter-Keyboard (commonly mapped at address 01F hexadecimal) |
| Unit record device | IBM 2540 Reader-Punch (addresses 00C & 00D) |
| Line printer | IBM 1403 Printer (address 00E) |
| Disk storage | two IBM 2311 Magnetic Disk Drives (units 190 & 191) each with 5 MB capacity |
| Tape storage | two IBM 2415 Magnetic Tape Drives (units 180 & 181) |
To minimize expenses, certain CPU functionalities such as the interval timer and storage protection were offered as optional features.
System software
Available Operating System choices included:
- BPS – Basic Programming Support
- BOS – Basic Operating System
- TOS – Tape Operating System
- DOS – Disk Operating System
Introduced in 1965, BPS functioned primarily as a collection of utilities and compilers distributed on punched cards, requiring neither disk nor tape drives.
The BOS system demanded a disk drive but was capable of running on the smallest Model 30 configuration with only 8K of memory.
For both DOS and TOS, a minimum of 16 KB memory was necessary.
TOS, which depended on tape drives but not disks, shared much of its codebase and documentation with IBM’s DOS/360. It underwent fourteen revisions before being phased out as disk storage became more economical.
DOS gained widespread popularity on the Model 30 and remained extensively utilized across smaller IBM systems into the early 1980s.
While BOS incorporated a spooling mechanism for managing print queues, DOS did not have this feature until the late 1960s when an add-on named POWER was introduced.
Programming languages
Commercial software development on the System/360 predominantly employed COBOL, RPG, and Assembler. For scientific and engineering computations, Fortran was commonly used, alongside a subset compiler for PL/I known as PL/I(D). COBOL programs originating from other platforms could be recompiled for the System/360; however, adjustments to the INPUT-OUTPUT SECTION were necessary to align with System/360 device assignments.
Compatibility features
Ensuring compatibility with prior systems played a crucial role in driving hardware sales. Although the System/360 instruction set itself lacked backward compatibility, IBM provided emulators to execute legacy software.
IBM 1400 series emulation
By employing additional Compatibility Feature hardware combined with DOS/360 software, object code from IBM 1401/1440/1460 systems could operate in emulation mode with little need for rewriting. This capability was widely adopted by many customers to maintain support for older applications.
IBM 1620 emulation
The 360/30 possessed the capability to replicate the IBM 1620; however, this feature held less significance due to the following reasons:
- The IBM 1130 was introduced as the successor to the 1620.
- The majority of applications on the 1620 were written in Fortran, and these programs could be adapted for execution on the System/360 platform.
External links
- IBM archives
- IBM Card Capacitor Read-Only Store (CCROS) at the Computer History Museum
- Card from a Card-Capacitor Read-Only Store (CCROS), System/360 Model 30
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