What does PUSHJ do? (was Re: was "C on the PDP-8 ... ")

It's a PAL macro for push and jump and there is a return and pop. Come from the fact that PDP-8 put the return address at the called location and the next location is executed. MAIN ; do something JMS I,FOO ; more something FOO ; return stored here Add I,BLAH JMP I,FOO In PDP-8 the I in the addressing is use the contents at the address pointed to. Now if you want recursion You call a standard call and return routine (COSMAC 1802 requires this too). The routine gets the target address(work routine) and the return adddress (caller) and saves the return and dispatches to the work routine. The reverse is done to get back to the originating caller. In the PDP-8A and 6120 there are IOTs to push/pop the ACC on the stack and and also there are a set of push/pops for the PC register. The general PDP-8 archetecture is very unique in that IOTs are really a handoff of the CPU core and registers to the device. So it's possible for IO hardware to get the PC save it and add a number to it and put it in the PC. The EMA is IO hardware. Oh, and they can also ouput/input from the ACC too. However the PDP-8 programmer is less likely to use a stack than a flavor of computed jump or call. PDP-8 is good candidate for an state machine coding. Allison