Hi To supplement Charles Mills response to your question on why Entry and Exit conventions are used in Assembler I will add the following.
Think single processor for a moment, as that was or starting point for System/360. The Operating System was designed to sustain multiple operations based on interrupts. When a program issues an I/O it relinquishes control of the processor, and allows another program to execute on that processor. When the I/O is complete an interrupt is issued to inform the program that lost control, that it can now resume processing. It may not be a simple as that but it conveys the general picture. If there are multiple programs alternately using the processor without completing then they need to be able to pick up where they left off, i.e. the state of the processor needs to be preserved across interrupts. The General Purpose Registers are part of the processor, so therefore their contents needs to be preserved. At a system level this is done by the interrupt handlers, but within an application it is our responsibility. If a program calls a subroutine, the main program looses control and the sub-routine gains control until its logical end point. The sub-routine will be using the same set of registers as its parent, so again they have to be preserved and restored. The process described above is also true for high level languages but it is done under the covers. The entry and exit sequence you listed are the historic ways of achieving the preservation goal, although I would use BASR instead of BALR. Check out the difference in the Principles of Operation. IBM provides SAVE and RETURN to aid this common function. There is now a stack capability to alternative Entry and Exit protocols might be BAKR and PR. Even with stacking the BALR or BASR followed by USING are still required to establish the correct addressability of this routine. I hope this is of some use. Kind Regards - Terry Director KMS-IT Limited 228 Abbeydale Road South Dore Sheffield S17 3LA UK Reg : 3767263 Outgoing e-mails have been scanned, but it is the recipients responsibility to ensure their anti-virus software is up to date. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
