Template - A new Idea

So - you want to describe a new idea? Great.

We give some guidelines how to do it. Below we describe the idea of a DUMP utility with parts that we find useful. This text is written in Wiki syntax. You are invited to make experiments in the playground.

Just edit this text and adapt it to your idea. If in doubt you can always save a draft and come back later.

Thanks for your contribution,

The Project Forth Works Team

DUMP memory algorithm: display content of main memory

STATE idea

Forth has two states: interpreting and compiling, depending on the content of the variable STATE . There are surprisingly many problems with this simple concept. These are caused by the fact that we may want to do interpretation during compilation, or may desire to start new definitions (i.e. use compile state) when we have not finished compiling the current definition yet. Compounding the problem is that changing the value of STATE all the time is cumbersome, and therefore most Forth have so-called IMMEDIATE words that

DUMP Implementation

One way to implement DUMP is to iterate in a loop line by line through memory. Say you want to display 16 bytes in each line, then this loop will start at the start address and increment the address by 16 in each loop iteration. (If you want a different number of bytes in each line, adjust accordingly).

When you display memory in a single line you first output the current address and then have two loops that run one after the other iterating both from 0 to 15. The first loop outputs bytes with two hexadecimal digits (or in decimal, or whatever you intend) and the second loop outputs the individual bytes as ASCII characters.

As some characters might control the output in a special way (so called control character such as 07 bell, 0A linefeed, 0C formfeed) it is wise to just output a period instead of the actual character, in order to get a well formatted display.

Pseudo code for the DUMP implementation

What Pseudo code is.

Function: dump-line ( address -- )
  output address (possibly right aligned)
  output ":" and some space
  LOOP i from 0 to 15:
     output byte in memory at (address+i) as two hexadecimal digits (or in another radix if desired)
     output some space
  LOOP i from 0 to 15:
     output byte in memory at (address+i) as an ASCII character, "." if that character is a control character (byte<32)
Function: dump ( address length -- )
   WHILE length is positive:
        dump-line at address
        increase address by 16
        decrease length by 16

Minimal Forth implementation of DUMP:

You can find a Minimal Forth implementation of DUMP in dump-Minimal_Forth.fs.

You can use it as follows:

Create x 100 allot  ok
x 100 dump
03D694: 05 61 6C 6C 6F 74 08 00 DF 14 00 00 C1 00 08 00   .allot..?...?...
03D6A4: 1F 18 00 00 C1 00 08 00 6F 10 00 00 C1 00 08 00   ....?...o...?...
03D6B4: 4F 10 00 00 C2 00 08 00 1F 13 00 00 C2 00 08 00   O...?.......?...
03D6C4: 8F 30 00 00 C2 00 08 00 DF 14 00 00 C2 00 08 00   ?0..?...?...?...
03D6D4: 1F 18 00 00 C2 00 08 00 6F 10 00 00 C2 00 08 00   ....?...o...?...
03D6E4: 4F 10 00 00 C3 00 08 00 8F 30 00 00 C3 00 08 00   O...?...?0..?...
03D6F4: 5F 13 00 00                                       _... ok

As the original Minimal Forth has no output facility other than emit and .s (specifically no number formatting and no . or .r) this implementation seems to be over complicated.

We extended Minimal Forth to GenericForth to get a more useful small Forth.

A DUMP utility in GenericForth can be found in dump-GenericForth.f This example is factored using the pseudo code description. The character output has been factored into the useful word PEMIT ( char – ) too.

Various DUMP Implementations

Other DUMP implementations can be found at the end of this description.

Your system might lack right justified number output or even BASE for printing numbers in other radix systems. The sample implementation in twomoredumps.f show how to circumvent this.

Background information

More about the DUMP utility can found at the Wikipedia page for hexdump

Some Forth DUMP implementations display a fixed amount of bytes and leave the updated address on the stack so that you can invoke DUMP repeatedly to display successive regions of memory.

Possible pitfalls with DUMP

Some systems have hardware memory protection that is triggered if you access memory outside the reserved area. The dump utility can do so by trying to show this forbidden memory. Triggered memory protect might stop the current process and terminate your session. If necessary a suitable test for the validity of used addresses might be reasonable on such systems so that dump can issue a normal error message (or display dummy data) in theses cases and leave the system / session otherwise intact.


Alternative Implementations

You have another approach to DUMP?

Please add it at the end of this document.

Melden Sie sich an, um einen Kommentar zu erstellen.
pfw/yournewidea.txt · Zuletzt geändert: 2024-05-31 22:57 von mhx