Editing IBM System/36 (section)

==System architecture==

===Processors===
S/36s have two 16-bit processors, the CSP or Control Storage Processor, and the MSP or Main Storage Processor. The MSP is the workhorse; it performs the instructions in the computer programs. The CSP is the governor; it performs system functions in the background. Special utility programs are able to make direct calls to the CSP to perform certain functions; these are usually system programs like $CNFIG which is used to configure the computer system. As with the earlier System/32 and System/34 hardware, the execution of so-called "scientific instructions" (i.e. [[floating-point arithmetic|floating-point operations]]) is implemented in software on the CSP.<ref>{{cite web|url=http://www.bitsavers.org/pdf/ibm/system36/SC21-7908-3_System_36_Programming_with_Assembler_Jan86.pdf|title=System/36 Programming with Assembler|date=January 1986|access-date=2021-12-22|publisher=IBM|website=Bitsavers}}</ref>

The primary purpose of the CSP is to keep the MSP busy; as such, it runs at slightly more than four times the speed of the MSP. The first System/36 models (the 5360-A) have a 4&nbsp;MHz CSP and a 1&nbsp;MHz MSP. The CSP loads code and data into main storage ''behind'' the MSP's program counter. As the MSP is working on one process, the CSP is filling storage for the ''next'' process.

The 5360 processors came in four models, labeled 5360-A through 5360-D. The later "D" model is about 60 percent faster than the "A" model.

===Front panel===

The 5360, 5362, and 5363 processors had a front panel display with four hexadecimal LEDs.  If the operator "dialed up" the combination F-F-0-0 before performing an [[IBM System/360 architecture#Initial Program Load|Initial Program Load]] (IPL, or [[Booting|system boot]]), many [[diagnostics]] were skipped, causing the duration of the IPL to be about a minute instead of about 10 minutes. Of course part of the IPL was typically keysorting the indexed files and if the machine had been shut down without a "keysort" (performed part of the P S (or STOP SYSTEM) then depending on the number of indexed files (and their sizes) it could take upwards of an hour to come back up.

===Memory and disk===

The smallest S/36 had 128K of RAM and a 30 MB hard drive.

The largest configured S/36 could support 7MB of RAM and 1478MB of disk space. This cost over US$200,000 back in the early 1980s. S/36 hard drives contained a feature called "the extra cylinder," so that bad spots on the drive were detected and dynamically mapped out to good spots on the extra cylinder. It is therefore possible for the S/36 to use more space than it can technically address. Disk address sizes limit the size of the active S/36 partition to about 2GB; however, the Advanced/36 Large Package had a 4GB hard drive which could contain up to three (emulated) S/36s, and Advanced/36 computers had more memory than SSP could address (32MB to 96MB) which was used to increase disk caching.

Disk space on the System/36 was organized by ''blocks'', with one block consisting of 2560 bytes. A high-end 5360 system would ship with about 550,000 blocks of disk space available. System objects could be allocated in blocks or records, but internally it was always blocks.

The System/36 supported [[memory paging]], referred to as "swapping".<ref>{{cite web|url=http://bitsavers.org/pdf/ibm/system36/SC21-9019-5_System_36_Concepts_and_Programmers_Guide_198706.pdf|title=System/36 Concepts and Programmer's Guide|date=June 1987|access-date=2021-05-01|publisher=IBM}}</ref>

===Software===

[[File:System-36-ssp-main-menu.png|thumb|300px|System/36 SSP Main Menu]]

{{main|System Support Program}}

The [[System Support Program]] (SSP) was the only [[operating system]] of the S/36. It contained support for multiprogramming, multiple processors, 80 devices, job queues, printer queues, security, [[indexed file]] support, and fully installed, it was about 10MB. On the Advanced/36, the number of workstations/printers was increased to 160. In the Guest/36 environment of certain OS/400 releases, up to 216 devices were supported.

The S/36 could compile and run programs up to 64 kB in size, although most were not this large. This became a bottleneck issue only for the largest screen programs. With the Advanced/36, there were features added to the SSP operating system including the ability to call other programs from within. So a program that was say 60 kB could call another program that was 30kB or 40KB. This call/parm had been available with third-party packages on the System/36 but not widely used until the feature was put in 7.1 and 7.5 of SSP on the Advanced/36.