Address Spaces and Processes

with examples from Bull GCOS7 and other computers


2002 Jean Bellec

Before stored programs computers were born, programs were implemented through a connections panel, a matrix of wires that defined functions like code conversions, filed selections etc… derived from first tabulators of the first half of 20th century. A new feature was created using the same technology, the "programming step" where a sequence of instructions could be executed at the rhythm of the mechanical clock. Those “programs” were not modifiable dynamically and their address space was limited to a few registers. At the end of that small procedure operating on large amount of punched cards (later emulated on first magnetic media), the program had to be manually changed by introducing a new connections panel. That was the era of punched cards tabulators and of their associated “calculators”, such as the Gamma 3, but also the first scientific computers (ASCC, ENIAC)


Then came the stored program computer and the so-called Von Neumann architecture, where programs are located in writable storage. That was initially considered as a genius decision because programs were able to be self-modified and that architecture raised a lot of speculation on the possibility of an artificial intelligence. But, in fact, apart the design of innovations like LISP, that feature came nowhere because the bulk of the market was processing large amount of data for scientific usage or business data processing. Stored programs computers had to wait the language compilers (in the late 1950s) to compete usefully against connections panels. The addressing of the storage was non-uniform in many machines, because a large part of the address space was within a secondary storage (usually a drum).
Later, all addressable memory was included in the main storage and the magnetic peripherals were storing just files (an emulation of old card decks).


At that time, there was an address space that was the physical memory and that belong to single execution of a program (i.e. containing the program and the registers). It was the time of Uni-programming (from IBM 704 to 1401). In those machines, the application program was booted on a bare machine after being linked to some system subroutines. Later, at the end of the 1950s, when a job terminated (almost) normally, an embryonic operating system, sometimes called monitor, took the place of the terminating job and loaded automatically the following job, decreasing the burden on the operator.

Among the functions linked to the user application, a piece of software, called an overlay manager insure a multiplexing of the (physical) address space on several pieces of code loaded in memory at different time. Those overlays had to be carefully planned by the programmer to allow the sharing of a part of the address space between successive overlays.


Then there was a thing (later considered as a process), the supervisor –sometimes also called monitor-, that allocated a separate distinct address space to each of several program executions. It was the time of Multiprogramming (GECOS II, OS/MFT…). Those address space were allocated for the time of a job and then disappeared.


Then the minicomputer appeared. A human controlled the resources via commands. Each command was allocated the totality of the physical memory. When it was the execution of a user program, it was likely that the machine has to be rebooted. Those systems reigned on part of the world from IBM 1620 to CP/M and MS/DOS.


Then the time-sharing was born (CTSS, CP/CMS time). Each user ought to feel as he had for himself a minicomputer. Each user was given a virtual processor. Each user was running a succession of commands (process) operating in a virtual address space that looked as a subset of the physical address. When a command was not in execution the “shell” was in control of the virtual processor.

The concept of daemon was also introduced as special virtual users that do things like copying cards into files , printing files…


Then came MULTICS. Instead of defining a limited virtual space to each user, it gave him the whole world of file system. More exactly, each user had a dynamic virtual space where segmentation was mapping the useful part of the file system. In early MULTICS, the user had a virtual single processor. He was mapped on a process.

Daemons also were considered as processes.

 The SABRE system, born at the same time as CTSS, had to handle many users, much more than does a time-sharing system. But all the programs were American Airlines or IBM written and presumably debugged. The tasks of each terminal were sharing a large common address space (procedures and windows in the data base), they had only an additional limited private address space for working (stack and context of the transaction). However, it was somewhat unpractical to allocate a virtual processor to each of the reservation clerk. So, there was mapping of those tasks on a handful of processes. 

The idea of letting an application program to do parallelism with several (virtual or real) processors probably came from the scientific world (tbc). There was an argument that the complexity of such an operation came from the lack of appropriate clauses in programming languages. So was born PL/1 that created tasks forking and synchronization. Address spaces of subtasks were defined within the primary task address space by means of the block structure of the language source program. PL/1 introduced a concept that would allow apparently to  program easily a transaction system, like SABRE. However, there was no way of protecting a task against another, but by checking the program correctness.
In IBM OS/MVT and its successors, IBM had some trouble to map tasks with “virtual processors”. It prefers to support PL/1 via the concept of subtasks encapsulated into their main tasks.
In CICS also, derived conceptually from SABRE, the simultaneous transactions were handled by subtasks sharing the same CICS address space.
IBM was quite late to support multiprocessors in MVS and in CICS and that made GCOSIII a great system…

UNIX came in the early 1970s and originally it was a minicomputer with a Multics-like file system. When UNIX became multi-user it allocated to each user a single address space (initially only real, then virtual) distinct from the file system. The word “process” became equivalent of a user and of that address space. Daemons were also processes created by God (the root) or by men (users). That was the paradigm of all UNIX systems until 1990. The kernel of most UNIX systems had been monolithic and not structured before a few micro-kernels like CMU MACH and Chorus were designed in the late 1980s. Micro-kernels implemented what they called threads to structure asynchronous actions in the kernel not just to improve the reliability by better structuring, but also to be able to distribute the kernel on separate processors (including NUMA systems). Micro-kernel architecture was challenged on the name of efficiency and OSF did not pursue the MACH approach.

The availability of UNIX on multiprocessor systems (including supercomputers) raised again a demand for multi-threading several tasks on behalf on a single user. There was, for some time, SMP-safe UNIX systems where all system commands and services were controlled by a big system lock, but progressively real multi-threading became available on UNIX (from the 1990s).

UNIX on Intel x86 (not only Linux) architecture used almost exclusively the linear address space of model 386 and did not used the segmented space , a 286 legacy. A portability objective was the prime motivation behind that decision but 286 segmentation got a negative image by the design flows of the 286. A revisiting of the UNIX design would have been necessary to use segments anyway.


Windows 16-bits did multi-threading on behalf on a single user, all threads sharing the same address space and bumping against each other. Win32 allocated a single address space to each asynchronous command launched from the shell. Windows/NT (and succesors) was supporting, at least conceptually, several users (i.e. several shells).

It also allowed multithreading but, as UNIX, it  is sharing the same address space between all the threads of the same program.


Open systems entered the transaction processing market via the client/server approach and avoided, at least temporarily, a difficult software architecture dilemma. Transactions are performed in a server  (a –potentially multi-threaded- daemon ) that accesses a data-base and maintain journals of updates. The server address space had relatively modest size. Obviously, the database is not directly included in that address space.

The state of the transaction is stored in the user computer acting not only as a terminal but also keeping the transaction context. The solution works well when the loss of that storage in the user computer does not impact the business of the owner of the database. It may require complex –at programmer level- recovery procedures when the transaction is “mission critical”. Many client/server operations were not making better than the 1960s first TP systems.


GCOS8 is a derivative of a traditional architecture born in the 1960s. In the mid-1970s it was extended with a capability mechanism implemented through a segmentation mechanism. Unhappily, the new system architecture has not been used coherently. Much of the batch system was unchanged and did not use it. The transaction processing eventually used a subset of the architecture to provide reentrance of TPR. The linkage conventions were not uniformly implemented and stayed dependent on languages. The multi-threading was progressively extended, but the support of several processors by a single TP subsystem was only a little bit earlier than IBM’s.


GCOS64 (the future GCOS7) was born in the 1970s (around the same time as UNIX). Its marketing objectives were oriented towards traditional mainframes applications and excluded time-sharing technical applications. However, high reliability and efficient support of complex applications such as simultaneous operation of native mode and emulators led us to propose a structured system architecture using several features borrowed from Multics. Level 64 got from Multics a hardware segmentation mechanism, somewhat constrained by the 32-bits word and a ring-protection mechanism. In addition, it froze in firmware a micro-kernel mechanism that architectured the concept of “process” (including their synchronization) and a segmented address space, divided into the public part, the dynamically linked part, the “process group” shared part and the private part. The new concept was the  “process group” that covers miscellaneous entities like the system itself, the non-permanent daemons, the transaction subsystems, and the emulators. It allowed eventually to have a UNIX port operating as an emulator. Conventional job steps in execution were loaded as process groups, although few of them were multi-threaded in several processes. But a transaction server was usually multi-threaded as two dozens of “processes”, although thousands of transactions were mapped by the server into those “processes”.
A big difference with Multics, however, was that the file system remained distinct from the address space, the “buffers” segments being windows into the file space. Most of files were “structured” and accessed exclusively through an “access method” , like IBM OS or GCOS.
Time-sharing was eventually implemented. A listener with one “process” by logged-in user was interpreting the shell –as an extension of the operator control- and most of the commands lead to the spawning of a “process group” with all the capabilities of a batch job step.

The limited address space due to the 32-bits words have been for long bypassed by the software implementations that adopted more and more a server approach that reduced the needs in address space. Bull and NEC had designed in the late 1980s an extension of the architecture named XSA that put aside most of those limits. Unhappily, Bull had already decided to wind down GCOS7 in front of non genuine architectures and NEC alone did only use XSA on a limited amount.  It should also be recognized that network computer transfer the software emphasis on distinctive address spaces addressing a global file system (referenced by URLs). Each computer space became only a cache for software modules, documents and windows inside databases. Main frame computers evolve towards a cluster of servers run under a variety of operating systems frequently invisible to end users and even to programmers. The last avatar of GCOS7 will be to operate, emulated on Intel IA-64 hardware, in a cluster of commodity hardware. The reliability offered as a transaction server by its genuine architecture will be kept, while many functions  (essentially those related to interactive processing) are moved to the several varieties of Open architectures.


The discussion of many features of the software architecture is still interesting as a school case. New operating systems are sometimes reinventing the wheel and a better knowledge of their ancestors may help young designers. It is not certain that such discussions have an economic impact. If GCOS64 architecture has been maintained secret by Honeywell management in 1970-1974, it was not that management estimated that its value was great; it was because Honeywell believed that the software architecture features were only local mechanisms to run applications. They eventually became right, but they ignored that a not so different architecture (Intel 386's) invented 10 years later has succeeded to dominate the world and has made a small semiconductor company a quasi-monopoly during more than two decades.