Systems Programming Language

Programming language by Hewlett Packard
the HP programming language

Systems Programming Language, often shortened to SPL but sometimes known as SPL/3000, was a procedurally oriented programming language written by Hewlett Packard for the HP 3000 minicomputer line and first introduced in 1972. SPL was used to write the HP 3000's primary operating system, Multi Programming Executive (MPE). Similar languages on other platforms were generically referred to as system programming languages, confusing matters. Originally known as Alpha Systems Programming Language, named for the development project that produced the 3000 series, SPL was designed to take advantage of the Alpha's stack based processor design. It is patterned on ESPOL, a similar ALGOL derived language used by the Burroughs B5000 mainframe systems, which also influenced a number of 1960s languages like PL360 and JOVIAL. Through the mid 1970s, the success of the HP systems produced a number of SPL offshoots. Examples include ZSPL for the Zilog Z80 processor, and Micro SPL for the Xerox Alto. The later inspired Action! for the Atari 8 bit family, which was fairly successful. The latter more closely followed Pascal syntax, losing some of SPL's idiosyncrasies. SPL was widely used during the lifetime of the original 16 bit version of the HP 3000 platform. In the 1980s, the HP 3000 and MPE were reimplemented in an emulator running on the PA RISC based HP 9000 platforms. HP promoted Pascal as the favored system language on PA RISC and did not provide an SPL compiler. This caused code maintenance concerns, and 3rd party SPL compilers were introduced to fill this need.

Hewlett Packard introduced their first minicomputers, the HP 2100 series, in 1967. The machines had originally been designed by an external team working for Union Carbide and intended mainly for industrial embedded control uses, not the wider data processing market. HP saw this as a natural fit with their existing instrumentation business and initially pitched it to those users. In spite of this, HP found that the machine's price/performance ratio was making them increasingly successful in the business market. During this period, the concept of time sharing was becoming popular, especially as core memory costs fell and systems began to ship with more memory. In 1968, HP introduced a bundled system using two 2100 series machine running HP Time Shared BASIC, which provided a complete operating system as well as the BASIC programming language. These two machine systems, collectively known as HP 2000s, were an immediate success. HP BASIC was highly influential for many years, and its syntax can be seen in a number microcomputer BASICs, including Palo Alto TinyBASIC, Integer BASIC, North Star BASIC, Atari BASIC, and others. Designers at HP began to wonder "If we can produce a time sharing system this good using a junky computer like the 2116, think what we could accomplish if we designed our own computer." To this end, in 1968 the company began putting together a larger team to design a new mid sized architecture. New team members included those who had worked on Burroughs and IBM mainframe systems, and the resulting concepts bore a strong resemblance to the highly successful Burroughs B5000 system. The B5000 used a stack machine processor that made multiprogramming simpler to implement, and this same architecture was also selected for the new HP concept. Two implementations were considered, a 32 bit mainframe scale machine known as Omega, and a 16 bit design known as Alpha. Almost all effort was on the Omega, but in June 1970, Omega was canceled. This led to an extensive redesign of Alpha to differentiate it from the 2100's, and it eventually emerged with plans for an even more aggressive operating system design. Omega had intended to run in batch mode and use a smaller computer, the "front end", to process interactions with the user. This was the same operating concept as the 2000 series. However, yet another 2000 would not be enough for Alpha, and the decision was made to have a single operating for batch, interactive and even real time operation. To make this work, it needed an advanced computer bus design with extensive direct memory access (DMA) and required an advanced operating system (OS) to provide quick responses to user actions. The B5000 was also unique, for its time, in that its operating system and core utilities were all programmed in a high level language, ESPOL. ESPOL was a derivative of the ALGOL language tuned to work on the B5000's, a concept that was highly influential in the 1960s and led to new languages like JOVIAL, PL/360 and BCPL. The HP team decided they would also use an ALGOL derived language for their operating systems work. HP's similar language was initially known as the Alpha Systems Programming Language. Alpha took several years to develop before emerging in 1972 as the HP 3000. The machine was on the market for only a few months before it was clear it simply wasn't working right, and HP was forced to recall all 3000's already sold. It was reintroduced in late 1973 with most of its problems having been fixed. A major upgrade to the entire system, the CX machine, and MPE C to run on it, reformed its image and the 3000 went on to be another major success during the second half of the 1970s. This success made SPL almost as widespread as the 2000 series' BASIC, and like that language, SPL resulted in a number of versions for other platforms. Notable among them was Micro SPL, a version written for the Xerox Alto workstation. This machine had originally used BCPL as its primary language, but dissatisfaction with its performance led Henry Baker to design a non recursive language that he implemented with Clinton Parker in 1979. Clinton would then further modify Micro SPL to produce Action! for the Atari 8 bit family in 1983. HP reimplemented the HP 3000 system on the PA RISC chipset, running a new version of the operating system known as MPE/iX. MPE/iX had two modes, in "native mode" it ran applications that had been recompiled for the PA RISC using newer Pascal compilers, while under "compatible mode" it could run all existing software via emulation. HP did not supply a native mode compiler for MPE/iX so it was not an easy process to move existing software to the new platform. To fill the need, Allegro Consultants wrote an SPL compatible language named "SPLash!" that could compile to original HP 3000 code to run within the emulator, or to native mode. This offered a porting pathway for existing SPL software.

The classic HP 3000 organized physical memory into 1, 2, 4, 8 or 16 banks of 64K (65536) 16 bit words (128K bytes). Code (shared, non modifiable) and data were separate from each other and stored in variable length segments of up to 32K words each. Within a process, data addresses such as pointers were 16 bit offsets from a base regsiter (known as DB), or relative offsets from a pointer register (Q or S) that resulted in a valid address within the process's data area (called the stack). While primarily 16 bit word oriented, the system supported addressing of individual bytes in an array by using the word address where the byte was stored shifted left 1 bit then adding 0 to access the upper byte, or 1 for the lower byte. Thus byte addresses were separate and distinct from word addresses and the interpretation of an address was purely contextual. This proved to be quite troublesome and was the source of numerous bugs in both system and user code. Care had to be taken to treat byte addresses as 16 bit unsigned (that is, type LOGICAL) when using them in, for example, length computations, because otherwise a byte address of 2^16 or more would be treated as a 2s complement signed value resulting in erroneous calculation of lengths or offsets. An individual process had access to up to 254 code segments of up to 32K words each. The code segments were divided into two domains: the first 191 were "system" segments shared by all processes, and the remaining 63 were "user" segments shared by all running the same program. Control transfer specified either an routine number within the current segment, or an external segment number and a routine number within it. A small table at the end of the segment provided the entry point address for the routine. A process's code acted on data in the stack, a single private segment also up to 32K words in size. Unlike stacks in other architectures, the HP 3000 stack was used for process globals, state preservation, procedure locals (supporting nested calls and re entrancy), and numeric computation/evaluation. The operating system provided facilities for access to additional memory based (non stack) data segments, but these were not natively addressed by the instruction set and so the program was responsible for moving data from and to such "extra data segments" as necessary. SPL included a variety of support systems to allow programs to be easily segmented and then make that segmentation relatively invisible in the code. The primary mechanism was to use the compiler directive which defined which segment the following code should be placed in. The default was , but the programmer could add any number of additional named segments to organize the code into blocks.

SPL included a "bit extraction" feature that allowed simplified bit fiddling. Any bit, or string of bits, in a word could be accessed using the syntax, where x and y were the start and end bit positions from 0 to 15. (Significantly, x and y must be constants known at compile time.) Thus returned the lower byte of the word storing A. This format could be used to split and merge bits as needed. Additionally, additional operations were provided for shifts and rotates, and could be applied to any variable with the , for instance .