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Load from the terminal input buffer to parameter stack

Why does this code not work?

TIB 10 ACCEPT
TIB SP@ 1 cells - 10 cmove

In that code I tried to enter a string and store it in the terminal input buffer and later store it on the parameter stack.

But with .S I see that does not work.

Solution

  • The parameter stack grows towards low memory

    The main problem with the sample code is that the parameter stack grows towards low memory. So the starting point for the destination of the copy should be at a higher memory address (inside the existing/defined parameter stack). So instead of

    TIB SP@ 1 cells - 10 cmove

    it should be:

    TIB SP@ 1 cells + 10 cmove

    Memory allocation for the string

    The next problem is that there is not enough storage for the string on the parameter stack. ACCEPT has left over one cell (four bytes on a 32-bit system), the actual number of characters. With sample input "user10181" (9 characters),

    TIB 10 ACCEPT

    results in:

    .S <1> 9  ok

    Forgetting about that extra element for the moment1, for the purpose of this elaboration, we allocate four cells on the parameter stack (the actual value, for example, 235, does not matter), 16 bytes on a 32-bit system:

    235 DUP DUP DUP

    The result of TIB SP@ 1 cells + 10 cmove is then:

    .S <5> 9 235 8241 541085779 541215060  ok

    We see that three of the four cells (each with four bytes) have been overwritten by cmove (as expected).

    TIB is overwritten by subsequent input from the terminal

    Unfortunately, our copied bytes are not as expected. Decoding the output for the three changed cells (that are in decimal), first from

    8241 541085779 541215060

    to hexadecimal:

    2031 20405053 20424954

    And then decoding as ASCII:

    20 31 20 40 50 53 20 42 49 54
   1      @  P  S     B  I  T

    And reversing (we had the high memory first and the test platform is little endian):

     "TIB SP@ 1 "

    That is the first ten characters of our second line, TIB SP@ 1 cells + 10 cmove. Thus it is clear that the terminal input buffer (TIB) is too temporary to be used in this case.

    The solution

    The solution to the third problem is to have all the code compiled before we ask for user input. For instance, put it into a word, inputOnStack:

    : inputOnStack TIB 10 ACCEPT 235 DUP DUP DUP TIB SP@ 1 cells + 10 cmove ;

    The result is then:

    inputOnStack user10181  ok
.S <5> 9 235 24881 942747697 1919251317  ok

    That corresponds to "user10181" and the tenth character is "a" (most likely from the "a" in inputOnStack).

    Test platform:

    • Raspberry Pi, model B.
    • Operating system: Raspbian, as installed by NOOBS 1.3.10, released 2014-09-09.
    • Gforth: version 0.7.0 (installed with sudo apt-get update; sudo apt-get install gforth)

    1. A more advanced version of the code could use the actual number of characters. In any case, it should DROPped one way or the other to balance the stack if this code is be integrated into other code.