"Expanded channel capacity and the ability to use the high-performance IBM 3330 disk storage under either Operating System (OS)orDisk Operating System (DOS) were ... among the factors significant to the Model 135's ...capabilities."
The IBM System/370 Model 135 was announced March 8, 1971,
the only 370 introduced that year. The 135 was IBM's fifth System 370, and it was withdrawn October 16, 1979.
Special features
Although microcode was not a uniquely new feature at the time of the 135's introduction, having been used in most System/360 models and in most System/370 models introduced so far, the ability to upgrade a system's microcode without changing hardware, by storing the microcode in read-write memory rather than read-only memory, was not common at that time.
The read-write memory containing the firmware was loaded from a "reading device located in the Model 135 console"; this allowed updates and adding features to the Model 135's microcode. The "reading device" was a built-in (read-only) floppy disk drive. The 145, introduced the prior year, also had this feature.
Optional features
The Model 135 was the last of the 370s to be introduced without Virtual memory. Four of the five could be upgraded. Unlike the 155 & 165, which required an expensive
hardware upgrade to add a DAT box (Dynamic Address Translation), the 135 & 145 could obtain their virtual memory upgrades from a floppy disk.
Microcode upgrades were also available to add "user-selected options such as
Customers of the 370/135 had a choice of four main memory sizes, ranging from 96K to 256K.
Other
The 370/135 was introduced as running "under either OS or DOS. Newer versions thereof (DOS/VS and OS/VS1) and Virtual Machine Facility/370 (VM/370) subsequently became available options once the 135's microcode was upgraded to support virtual memory. This was priced at $120,000 and came with "increased reloadable control store in addition to some power units." The upgrade could be done "in the field" and the resultant system was now deemed a 370/135-3.
↑Padegs, A. (September 1981). "System/360 and Beyond". IBM Journal of Research & Development. 25 (5). IBM: 377–390. doi:10.1147/rd.255.0377. In the initial System/360 models, microprograms resided in read-only storage, but in most later models read-write storage is used. - tables include model characteristics (Table 1) and announcement/shipment dates (Table 2). The S/370-155-II and -165-II are listed under the former but not the latter, because the upgraded systems were not formally announced as separate models. The "System/370 Advanced Function" announcement, including the -158 and -168, was the main public event.
↑"DOS/360: Forty years". Newsgroup:alt.folklore.computers. May 16, 2005. Retrieved 2023-10-02. No, you'll find no IMPL button on a System/360 Model 30. It's[sic] control store was Card Capacitive ROS. System/360's used a variety of read only control stores, so changing the microcode meant replacing parts (e.g., with parts that were personalized by punching holes in a card). Later models like the 85 and 25 had writable control stores, the 85 using a volatile SRAM array and the 25 using a part of core memory.
