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Print Hex

In this programming pearl Albert Nijhof shows how to print hexadecimal numbers with a given number of digits even if your Forth system does not provide pictured numeric output by means of <# # #> etc.

 1  \ Tools -- Formatted unsigned single hex number output, not using BASE
 2  \ an-17jan2022
 4  decimal
 5  : .1HX ( x -- )     \ Print last digit of x in hex
 6      15 and                  \ lowest nibble
 7      9 over < 7 and +        \ for A..F
 8      [char] 0 +   emit ;
10  : .NHX ( x n -- )   \ Print last n digits of x in hex
11      1 max 16 min >r                 \ x r: n
12      r@ 1 ?do dup 4 rshift loop      \ collect on data stack
13      r> 0 do .1hx loop space ;
14  \ ----- end of code -----
16  (*
17  Examples
18  decimal
19  19150 2 .nhx     \ CE
20  19150 3 .nhx     \ ACE
21  19150 4 .nhx     \ 4ACE
22  19150 8 .nhx     \ 00004ACE
24  \ .NHX version without DO-LOOP
25  : .NHX ( x n -- )   \ Print last n digits of x in hex
26      swap >r   1 max 16 min dup          \ n n r: x
27      begin 1- dup while r@ 4 rshift >r   \ collect on return stack
28      repeat drop
29      begin r> .1hx   1- dup 0= until drop space ;
30  *)
31  \ <><>


In order to print numbers you have to extract the digits one by one and print each digit. As typical written number representation starts with the most signficant digit but extracting is easier starting with the least significant digit, the order of digits needs to be reversed. This could be done by storing the digits either on the data stack or on the return stack and make use of their last in first out property.

Display a single digit: .1HX

The word .1HX (print 1 digit in hex, line 5-8) displays a single digit, the last digit of the unsigned number x.

Line 6 extracts the least significant nibble of x and ignores its more significant part.

To display that nibble as a character the word uses arithmetic with comparison results:

The phrase 9 over < in line 7 compares the nibble with 9. If it is greater than 9 then the phrase results in -1 (all bits set).
If the nibble is not greater than 9 (i.e. 0 to 9) the phrase results in 0 (all bits 0).

7 and extracts the least 3 bits. Now we have either 7 or 0, that we add to the nibble itself. This leads to the nibble+7, if nibble is greater 9 or to the original nibble value, if it is not greater that 9. This is to bridge the gap in the ASCII character sequence between '9' and 'A' as we see now.

If we look at the ASCII character sequence we see:

48  49  50  51  52  53  54  55  56  57     58  59  60  61  62  63  64     65  66
'0' '1' '2' '3' '4' '5' '6' '7' '8' '9'    ':' ';' '<' '=' '>' '?' '@'    'A' 'B' ...

There is a gap between the character '9' and the character 'A' which is 7 characters wide.

Line 8 adds the character value of '0' to the intermediate value. Nibbles 0 to 9 will be mapped to character '0' to '9'. Nibbles 10 to 15 will be mapped (bridging the gap) to character 'A' to 'F' as is required for hexadecimal display.

This character is eventually displayed by the emit at the end of .1HX (line 8).

Display n digits: .NHX

In order to display complete numbers the word .NHX (print n digits in hex) is defined (line 10-13). x is the number to display and n ist the number to digits to show.

Line 10 does some clipping so that n is always in the range 1 to 16. This avoids unpleasant suprises of seemingly endlesss output if the passed n happens to be outside that range.

To display complete numbers .NHX first iterates n-1 times (DO LOOP on line 12) and puts shifted numbers on the stack so that each numbers least significant nibble is on of te nibbles of x. The number with the least significant nibble of x first, the number with the most significant nibble of x on top of stack.

After that the nibbles on the stack are processed in reversed order. They are displayed via .1HX (DO LOOP, line 13) that runs n times.
.NHX ends by printing a space so that consecutive calls to .NHX will print number space separated.


If your system does not provide DO LOOP then you can rewrite the loops using BEGIN WHILE REPEAT or BEGIN UNTIL as you can see in line 27-29.

Line 26 does parameter clipping of n as before.

To avoid stack juggling the first loop (line 28) collects nibbles on the return stack. (That wouldn't have been possible with DO LOOP above as the loop parameters block the return stack.)

The second loop (line 29) retrieves the nibbles from the return stack and prints them with .1HX as above. When finished it drops the loop parameter. The obligatory space ends the number output (line 29).

Example output

Lines 18-22 show hot to use .NHX and what output you can expect. If the number of digits n given is less then the actual numbers of digits to completely display x then .NHX truncates the display showing only the least signiicant digits. If n is larger, then .NHX adds leading '0's.

uho 2022-01-20

en/pfw/print-hex.txt · Last modified: 2023-09-04 18:17 by uho