Editing PDP-8 (section)

===Subroutines===
The PDP-8 processor does not implement a [[stack (abstract data type)|stack]] upon which to store registers or other [[context (computing)|context]] when a [[subroutine]] is called or an [[interrupt]] occurs. (A stack can be implemented in software, as demonstrated in the next section.) Instead, the JMS instruction simply stores the updated PC (pointing past JMS, to the return address) at the effective address and jumps to the effective address plus one. The subroutine returned to its caller using an indirect JMP instruction that addresses the subroutine's first word.

For example, here is "Hello, World!" re-written to use a subroutine. When the JMS instruction jumps to the subroutine, it modifies the 0 coded at location OUT1:

<pre>
    *10                     / Set current assembly origin to address 10,
    STPTR,  STRNG-1         / An auto-increment register (one of eight at 10-17)

    *200                    / Set assembly origin (load address)
    LOOP,   TAD I STPTR     / Pre-increment mem location 10, fetch indirect to get the next character of our message
            SNA             / Skip on non-zero AC
            HLT             / Else halt at end of message
            JMS OUT1        / Write out one character
            JMP LOOP        / And loop back for more
    OUT1,   0               / Will be replaced by caller's updated PC
            TSF             / Skip if printer ready
            JMP .-1         / Wait for flag
            TLS             / Send the character in the AC
            CLA CLL         / Clear AC and Link for next pass
            JMP I OUT1      / Return to caller
    STRNG, "H               / A well-known message
           "e               /
           "l               / NOTE:
           "l               /
           "o               /   Strings in PAL-8 and PAL-III were "sixbit"
           ",               /   To use ASCII, we spell it out, character by character
           "                /
           "w               /
           "o               /
           "r               /
           "l               /
           "d               /
           "!               /
           015              /
           012              /
           0                / Mark the end of our null-terminated string (.ASCIZ hadn't been invented yet!)
</pre>

The fact that the JMS instruction uses the word just before the code of the subroutine to deposit the [[return address (computing)|return address]] prevents [[reentrant (subroutine)|reentrancy]] and [[recursion]] without additional work by the programmer. It also makes it difficult to use [[read-only memory|ROM]] with the PDP-8 because read-write return-address storage is commingled with read-only code storage in the address space. Programs intended to be placed into ROMs approach this problem in several ways:

*They copy themselves to read-write memory before execution, or
*They are placed into special ROM cards that provide a few words of read/write memory, accessed indirectly through the use of a thirteenth flag bit in each ROM word.
*They avoid the use of subroutines; or use code such as the following, instead of the JMS instruction, to put the return address in read-write memory:
<pre>
    JUMPL, DCA TEMP         / Deposit the accumulator in some temporary location
           TAD JUMPL+3      / Load the return address into the accumulator: hard coded
           JMP SUBRO        / Go to the subroutine, and have it handle jumping back (to JUMPL+3)
</pre>

The use of the JMS instruction makes debugging difficult. If a programmer makes the mistake of having a subroutine call itself, directly or by an intermediate subroutine, then the return address for the outer call is destroyed by the return address of the subsequent call, leading to an infinite loop. If one module is coded with an incorrect or obsolete address for a subroutine, it would not just fail to execute the entire code sequence of the subroutine, it might modify a word of the subroutine's code, depositing a return address that the processor might interpret as an instruction during a subsequent correct call to the subroutine. Both types of error might become evident during the execution of code that was written correctly.