I don't think there is an easy answer to before or after. It depends on the particular logic. I have been writing assembler code for (literally!) fifty years and I have to pause and think and think and think every time about code like this.
You need to take a pencil and put a little line between every two instructions and say "if I get interrupted here, and all this other code runs while one thread is between these two instructions, what happens?" And do that for every two instructions. Further, if you could get interrupted by another processor (which is almost certainly the case) you have to do the same thing for every *individual* instruction that is *not* atomic (LM/STM, MVC, etc.). Charles -----Original Message----- From: IBM Mainframe Assembler List [mailto:[email protected]] On Behalf Of [email protected] Sent: Monday, July 29, 2019 10:45 AM To: [email protected] Subject: Circular Queue Handling in Assembler Hi, . I have a program which obtains a Memory Chunk, which is carved into a queue of 256 byte fixed length entries. I could have used a Data space. . In 31 Bit storage is the control information for the 64Bit Memory Chunk queue. The 31 Bit storage control structure has the beginning address of the Memory Chunk, the Ending Address Of the Memory Chunk, the number of fixed length entries an Ordinal Number (INDEX) and some state data. . The ordinal Number is used to index into the Memory Chunk of fixed length entries (queue) and is incremented using Compare and Swap (CS). The program never searches the queue to find an available slot - it always appends to the next entry by incrementing the ordinal number using compare and swap.. . The memory chunk is used as a circular queue, meaning, when we reach the end of the queue (memory chunk), we resume by re-using the first entry at the top of the queue (wrap around). We all-ways add new/next entries by incrementing the ordinal number and indexing into our Memory Chunk. . Here's My concern - When we reach the end of the queue - the program needs to reset ther Ordinal Index back to 0. (so we can continue to add the next entry at the beginning of the queue).. Is it better to test/reset this Index number before or after adding the last entry in the queue ?Should there be a second Compare and Swap ? . . Paul * .
