What was the purpose of XCT on the PDP-10/EX on the S/360?

On Tue, Sep 2, 2014 at 4:05 PM, Kevin Keith <krfkeith at gmail.com> wrote: I am working with entirely different hardware and software, but I have encountered functionally equivalent execute instructions used to adapt the code to hardware variations; e.g. different models of the I/O controller had different ways of getting the value of a status register. There is some initialization code like: int get_status_instruction; initialize () { ... if (dev_type == model_a) get_status_instruction = 0040355000; // instruction to read status from model A else get_status_instruction = 004488730; // instruction to read status from model B ... } #define get_status xeq(get_status_instruction) code... status = get_status; So, the idea is that the overhead of the execute instruction is less then the over head of checking the dev_type every time you need the status, and is easily changed to handle additional variations since only the initialize routine needs to be modified. -- Charles