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Finding little-endian values of 24-bit numbers

Posted: Wed Oct 16, 2019 11:51 am
by mikefromca
I'm doing a project in which data is sent from the PC to my own custom hardware via a serial port.

I coded my own hardware so that any numeric parameters needs to be in little-endian format. But I'm finding the PC likes to deal with numbers in big endian format.

Nice thing with Qbasic (which seems to be the only programming language I know that supports this directly) is that I can use mki$ and cvi and friends to help me in my quest.

So as an example, say the required packet format my hardware expects is as follows:

Byte 1: Function number
Bytes 2-4: 24-bit data
Byte 5: checksum

So my qbasic program will convert the string-hex format of the data to ascii to send down the serial line. By string-hex I mean where it prints hex characters as string.

For example, If I wanted to use function number 10 and the 24-bit data is the 24 bit number 254, and the checksum is 20, then in my program, I would have it process the string:


Because 0A is hex for 10, EF0000 is little-endian 24-bit value for 254 and 14 is hex for 20.

Eventually I want to make these values not fixed. Say I want the user to input the 24-bit value in and he types 254 as the number

Somehow I want it to go down the serial line as data equivalent to qbasic's chr$(&HFE)+chr$(0)+chr$(0)

I'll write code to illustrate my point:

Code: Select all

'convert to output for serial
toser$=chr$(funcn%)+chr$(myvalue& AND 255)+chr$((myvalue&/256) AND &H100)+chr$((myvalue&/&H10000) AND 255)+chr$(checksum%)
;convert to user readable output
for n%=1 to len(toser$)
next n%
print bytes$
The above code would achieve what I want but in reality the data packet size is much larger and may involve more numbers.

If my number was a 32-bit number, I could maybe replace my thought of:

Code: Select all

... chr$(myvalue& AND 255)+chr$((myvalue&/256) AND &H100)+chr$((myvalue&/&H10000) AND 255)+chr$(chr$((myvalue&/&H1000000))
with this:

Code: Select all

But what would be best for a 24-bit number? I'm just trying to cut down on the use of chr$() and number divisions.

Re: Finding little-endian values of 24-bit numbers

Posted: Wed Oct 16, 2019 12:38 pm
by burger2227

Re: Finding little-endian values of 24-bit numbers

Posted: Thu Oct 17, 2019 11:16 pm
by mikefromca
I was thinking more endian on byte-level not bit level.

For example,

Converting &H0000FE to &HFE0000 programmatically with simple commands, not multiple for-next loops going through each ascii character.

Perhaps I should learn more 8086 assembly and use call absolute more.