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Humanist Archives: July 7, 2019, 5:59 p.m. Humanist 33.124 - 'bastard octal'?

                  Humanist Discussion Group, Vol. 33, No. 124.
            Department of Digital Humanities, King's College London
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        Date: 2019-06-30 12:35:29+00:00
        From: Gabriel Egan 
        Subject: 'Bastard octal'


I wonder if any HUMANIST can confirm my recollection
about a numbering system known as 'bastard octal' that
was popular in microprocessor programming in the 1970s.
I have failed to find any online references to it, but
my recollection is strong. In case it helps to jog anyone's
memory, I include below a description of 'bastard octal'
and the rationale for its use. I would be grateful
for any recollections of using this numbering system,
or pointers to printed or online references to it.


Gabriel Egan
De Montfort University


True octal numbering groups the binary digits in a binary
number into groups of three, starting at the right end.
Each group of three bits is represented by its equivalent
in the decimal range 0 through 7, so for example 001 is
1, 011 is 3, 110 is 6, and 111 is 7. For an 8-bit number
(a byte) this grouping-by-threes runs out of bits for
the third group, so that the last two bits (the
most significant, left-most bits) are represented by an
octal digit from 0 to 3 since the 11 is the highest
value that these 2 bits can take.

Thus the largest 8-bit number (11111111) is represented
in true octal as 377. The next largest binary number,
which requires 9 bits, is 100000000 and this can be grouped
from the right into three groups of three bits (100,
000, and 000) so its equivalent in true octal is 400.

Often in microprocessor applications it is desirable to be
able to specify a 16-bit number (typically a memory address)
as two 8-bit numbers, one representing the lowest 8 bits (the
"low byte") of the 16-bit number and another representing the
highest 8 bits (the "high byte") of the 16-bit number. (For
example, the 6502 and 8080 microprocessors require that the
16-bit operand of an instruction, such as a memory address,
is stated as two bytes, the first being the low byte and the
second being the high byte. True octal representation cannot
do this. We cannot divide 400 octal into a low byte and a high
byte because the digit "4" encodes the two most-significant
bits of the low byte and the least-significant bit of the high

In 'bastard octal' this problem is avoided by encoding separately
the two bytes within a 16-bit number separately. Instead of
collecting groups of three bits from the right all
the way along the 16 bits (with one bit, the 16th
bit on the left, forming the final group), 'bastard octal'
groups the low byte from the right as a group of three bits,
another group of three bits, and then a group of two bits,
and then groups the high byte from the right as a group of
three bits, another group of three bits, and then a group
of two bits.

Thus whereas in true octal the 16-bit number 0000 0001 0000 0000
is grouped as 0 000 000 100 000 000 and represented as
400 octal, in 'bastard' octal it is grouped as
00 000 001 00 000 000 and represented as 001,000 octal.
(The comma shows that we treated each byte separately.)
In 'bastard octal', we can specify this 16-bit number
as a pair of bytes and if required to give them in
low-byte-then-high-byte order we just reverse the
'bastard octal' numbers so that 001,000 is specified
as 000 followed by 001.


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