zeroed out

MRAB python at mrabarnett.plus.com
Wed Dec 12 14:35:26 EST 2018


On 2018-12-12 18:59, Dennis Lee Bieber wrote:
> On Wed, 12 Dec 2018 00:46:03 -0500, "Avi Gross" <avigross at verizon.net>
> declaimed the following:
> 
>>
>>All kidding aside, I note that some data that is stored in a fixed width has zeroes padding it all the way to the right. If you store the number 3 in binary as a byte, it tends to look like 00000011. Depending on how integers are stored, something similar can happen for longer stretches. But with decimals, we tend to not bother showing the empty areas that represent 0 times ever higher powers of 10.
>>
> 
> 	Original COBOL implementation performs arithmetic on BCD (more likely,
> packed BCD). With two decimal digits per byte (packed BCD), a common
> 32-digit number requires 16 bytes, or 4 longwords (given the size of
> processors in 1960). Since arithmetic is basically an array operation in
> BCD (process least significant digit, process next digit..., etc.) numbers
> required storage of the filler zeros.
> 
> 	Furthermore, of more significance, formatting of a BCD number for
> output merely required taking each BCD digit (unpacked) or nybble (packed)
> and ADDING the value of (ASCII) "0". Conversely, input of (ASCII) fixed
> width/filled numbers simply requires subtracting "0" from each digit to get
> the BCD value for it (followed by packing the right-side nybbles to get
> packed BCD)
> 
> 	The Intel 8080 includes a DAA (decimal adjust accumulator) instruction
> to "correct" an 8-bit value back to packed BCD. Adding two 2-digit BCD
> values using normal binary ADD/ADC would result in an incorrect binary
> value (the source wasn't binary, but the result isn't BCD), DAA would
> adjust that back to BCD, using a "half carry" flag to handle the internal
> digit split.
> 
Later processors have a DAS instruction, which is used after BCD 
subtraction.

The humble 6502 doesn't have DAA/DAS, but instead has a decimal mode flag.

There's a clever way to perform BCD addition on long BCD numbers 
(32-bit, 64-bit, whatever) without having to do it 1 digit (or 1 byte) 
at a time.


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