Re: [TUHS] Happy birthday, PDP-8!

Essentially, you pass parameters in memory, as a part of the code stream. Also, the PDP-8 certainly do have index registers. The first thing one must do is stop thinking of the AC as a register. The accumulator is the accumulator. Memory is registers. Some memory locations autoincrement when used indirectly, they are called index registers. That said, then. A simple example of a routine passing two parameters (well, three): First the calling: CLA TAD (42 / Setup AC with the value 42. JMS COUNT BUFPTR BUFSIZ . / Next instruction executed, with AC holding number of matching words in buffer. . Now, this routine is expected to count the number of occurances of a specific word in a memory buffer with a specific size. At calling, AC will contain the word to search for, while the address following the JMS holds the address, and the following address holds the size. The routine: COUNT, 0 CIA DCA CHR / Save the negative of the word to search for. CMA TAD I COUNT DCA PTR / Setup pointer to the address before the buffer. ISZ COUNT / Point to next argument. TAD I COUNT CIA DCA CNT / Save negative value of size. DCA RESULT / Clear out result counter. LOOP, TAD I PTR / Get next word in buffer. TAD CHR / Compare to searched for word. SNA / Skip if they are not equal. ISZ RESULT / Equal. Increment result counter. ISZ CNT / Increment loop counter. JMP LOOP / Repeat unless end of buffer. CLA / All done. Get result. TAD RESULT JMP I COUNT / Done. PTR=10 CNT=20 CHR=21 RESULT=22 Addresses 10-17 are the index registers, so the TAD I PTR instruction will autoincrement the pointer everytime, and the increment happens before the defer, which is why the initial value should be one less than the buffer pointer. Hopefully this gives enough of an idea, but unless you know the PDP-8 well, you might be a little confused by the mnemonics. As you can see, the return address at the start is used for more than just doing a return. It's also your argument pointer. Johnny