need help with rand function.

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Hayden

Joined: 06 Oct 2005
Posts: 132

Hayden 20 Mar 2007, 19:36

The com file below attempts to create a random 8.3 mode compatable filename. The problem is I can't figure out the best way to do the rand

My rand part has an obvious floor where may put the machine into an infanite loop.

I've thought of adjusting for this in a few ways;

* Implement a 'tap bit' or something ?
* if CL even then SHR 1 bit ?

Code:

ORG 100h
USE16

init:
    mov bx, BiosClockTick
    mov di, RandomDOSName
    xor cx, cx
    mov cl, 8d
rand:
    mov al, [bx]
redo:              ; redo: needs some sorting out
    rol al, cl
    and al, 127d
num:
    cmp al, '0'    ; check for a valid digit
    jb char        ; nope, so check for char
    cmp al, '9'    ; check digit between 0-9
    jb redo        ; yes
char:
    cmp al, 'A'    ; check for a valid char
    jb redo        ; nope, so redo
    cmp al, 'Z'    ; check char between A-Z
    ja redo
next:
    cld
    stosb
    loop rand
puts:
    mov dx, RandomDOSName
    mov ax, 0900h
    int 21h
    int 20h

BiosClockTick =  046Ch
RandomDOSName dq ?
TempExtention db '.TMP'
DOSTerminator db '$'

_________________
New User.. Hayden McKay.

20 Mar 2007, 19:36

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 20 Mar 2007, 19:52

To avoid infinite loop you could try getting "AL := AL MOD ('9'-'0'+'Z'-'A'+2)" and then mapping AL to the appropiate char (AL < 10 then AL := AL+'0' else AL := AL + 'A')

20 Mar 2007, 19:52

DOS386

Joined: 08 Dec 2006
Posts: 1900

DOS386 20 Mar 2007, 20:16

Quote:

avoid infinite loop you could try getting "AL := AL MOD ('9'-'0'+'Z'-'A'+2)"

AL := AL MOD 36 Laughing

Quote:

and then mapping AL to the appropiate char (AL < 10 then AL := AL+'0' else AL := AL + 'A')

Right ...

but there seem to be more problems:

Code:

    mov bx, BiosClockTick
    mov al, [bx]

Will this read from correct address at all ? Confused

And: even if you fix 2 previous problems, there is one more left:
the complete "randomness" depends of one single byte,
you will get mostly only 256 different "random" names Sad

Repeated reading from BiosClockTick will almost never give new
values ... the code will be far too fast as soon as you get rid of the
infinite loop Sad

You should use 2 bytes from BiosClockTick (the 2 highest bytes are bad
as well - mostly 0), read from there one time only ... yeah ...
only 65536 names possible ...

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

20 Mar 2007, 20:16

Hayden

Joined: 06 Oct 2005
Posts: 132

Hayden 21 Mar 2007, 12:45

Thanks very much for giving me some direction guys. I acctualy expected the code not to work when I wrote it.

I think I should use mov 0:[bx] , or an overide segment set to zero.
I have thought about doing a rand 0 -> 25 + 'a' approach. but that would limit name sto smallcase.

21 Mar 2007, 12:45

DOS386

Joined: 08 Dec 2006
Posts: 1900

DOS386 22 Mar 2007, 05:46

Quote:

to create a random 8.3 mode compatable filename

Quote:

doing a rand 0 -> 25 + 'a' approach. but that would limit name sto smallcase.

1. From 8.3, I assumed DOS - and DOS filenames are always UPPERCASE. Shocked

2. You randomness is insufficient already for (A...Z)&(0...9), you should
forget about lowercase for now.

_________________
Bug Nr.: 12345

Title: Hello World program compiles to 100 KB !!!

Status: Closed: NOT a Bug

22 Mar 2007, 05:46

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