The 9845A/S
The 9845A was the first model of the 9845 series. First announcement was in the Keyboard Magazine 77/3 and the 9845A sales brochure from 1977. The 9845A was introduced in the April 1978 issue of the HP Journal and appeared first in the HP Computer Catalog from 1978. It already contained most of the concepts, which characterized the whole series.
In order to understand the design of the 9845 series, one have to envision that in the late 70's, there were no desktop or personal computers as we use to know them. The computer scene was dominated by heavy and expensive systems like the IBM 370 or minicomputers like the HP 3000. CRT based terminals were already in use with those systems, but CRTs were not common with desktop computers. On the other side, there was an upcoming homecomputer scene, based on microprocessors like the Intel 8080, the Motorola 6800, the MOS 6502 or the Zilog Z80.
The situation from the HP point of view was very special. HP employees and many of their customers were engineers and scientists. HP was strong in the fields of measuring equipment, calculators and minicomputers. Surveys with their customers suggested that there was a need for high-end computing power at the desktop, with capabilities for complex calculations, and features like graphics, mass storage and printers. Those engineers and scientists were close to technology, but no computer specialists and had only limited system knowledge about computers.
Consequently, the system should be easy to use and at the same time as flexible and powerful as possible. So HP concentrated their knowledge from both the computer and the calculator engineering, and designed a highly integrated desktop system, and one of the very first workstations was born. The HP 9845A outclassed any home computer, and provided minicomputer power on desktop level.
Some design decisions, like using a built-in CRT, splitting the processing power between a language processor and an I/O processor, hardware support for floating point calculations, the use of function keys and a numeric keypad, or providing an I/O bus with extension modules for connecting peripheral, were exemplary for many subsequent systems.
The system was sold in two configurations:
- The 9845A, a basic configuration with 16K Bytes LPU R/W Memory, one tape cartridge drive and no internal printer, and
- the 9845S, an expanded version with two internal cartridge drives, a built-in thermal line printer, and 64K of LPU R/W memory, and graphics support.
The 9845A was priced at $11,500, whereas the 9845S was priced at $20,000. The number of 9845A units sold is unknown, however the vast majority of System 45 computers sold were 9845B models.
Some of the innovations of the 9845A were covered by the US patent No. 4180854 from 1979. Those innovations include the new CRT display, modifications of the 9825 hybrid processor, the memory address extension, the enhanced HP BASIC and the internal printer.
The 9845A was replaced by the 9845B in 1980.
In contrast to the 9845B, all of the 9845A OS was held completely in plugable ROM modules. Those modules occupied four slots in each ROM drawer and left only four slots for option ROMs, one of the reasons why they later moved on the RAM/ROM assemblies. The 9845A ROM modules can't be used with the 9845B ROM drawers and vice versa.
Language processor and peripheral processor were both situated on one single assembly, sharing one memory address extension and one dual port memory controller. In the 9845B, each processor was placed on its own assembly, together with its own memory controller and memory address extension. The 9845A memory address extension was designed for four memory blocks of 64 KBytes each, so the total memory space for both LPU and PPU was limited to 256 KBytes:
- block 0 was reserved for LPU R/W memory,
- block 1 for PPU system ROM, some I/O R/W memory, and PPU option ROMs,
- block 2 for LPU option ROMs and
- block 3 for LPU system ROM.
The memory address extension of the later 9845B was designed for up to 64 memory blocks with an extended address space of up to 4 MBytes.
There were two different 9845A R/W memory assemblies, one with 8 KBytes DRAM (used with the PPU) and one with 16 KBytes DRAM (up to four of those boards could be installed for the LPU). The later 9845B featured combined RAM/ROM boards with 32 KBytes DRAM for the PPU and 64 KBytes DRAM for the LPU, as well as R/W memory extension boards of 128 KBytes and 512 KBytes.
The 9845A motherboard was slightly different than that of the 9845B. keyboard, PSU, tape cartridge drives, the internal thermal printer and the CRT monitor were the same as those of the 9845B.
Most BASIC commands of the 9845B already work on the 9845A. Exceptions are CAT TO, ON KBD, OFF KBD, and KBD$. Therefore, BASIC programs are generally interchangeable, sometimes with some minor modifications. Unfortunately, the STORE format was changed between the 9845A and the 9845B, so programs first have to be SAVE'd on a 9845A and then loaded with GET at a 9845B. Files of type DATA and BDAT could be interchanged, all other file types not.
In general, binary routines developed for a 9845A will not run on a 9845B and vice versa. This is particulary caused by the different memory address extension and different OS routine jump vectors.
Technical Specifications
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More Information
Basic sales information about the 9845A is contained in the 9845A brochure from 1977. As background for the 9845A it is recommended to examine the April 1978 issue of the HP Journal (see here for download). Additional details for the system and the whole concept may be found in an article written by the developers of the 9845A in a compendium about computer structures and published at McGraw-Hill (look here for download) and, of course, in the patent mentioned above (look here for download).
There is no dedicated service or CE handbook for the 9845A, at least as far as I know. Although the differences between the 9845A and the 9845B are not that significant, and altough there already was a test ROM for the 9845A, I wonder how the service support for those systems was organized between 1978 and 1980.