Packed six-bit characters - Revision history

TArntsen: Use NORD PL|NPL instead of NPL to avoid redirection, to help the wayback machine.

2015-03-24T12:45:30Z

Use NORD PL|NPL instead of NPL to avoid redirection, to help the wayback machine.

← Older revision		Revision as of 12:45, 24 March 2015
Line 16:		Line 16:

	=== S-group ===		=== S-group ===
	The [[File_Formats#BRF\|BRF]] format, i.e. compiled, non-linked ND-100 object code, stores symbols in packed 6-bit characters. These symbols are called S-groups. The symbol may be stored in two 16-bit words for BRF files generated by [[MAC]] and [[NPL]], and contain up to 5 characters, or stored in three 16-bit words for BRF files generated by language compilers like FORTRAN. In the latter case the maximum symbol length is 7 characters, even though technically there is room for 8 characters inside 3 words (16*3 = 48, and 48/6 = 8).		The [[File_Formats#BRF\|BRF]] format, i.e. compiled, non-linked ND-100 object code, stores symbols in packed 6-bit characters. These symbols are called S-groups. The symbol may be stored in two 16-bit words for BRF files generated by [[MAC]] and [[NORD PL\|NPL]], and contain up to 5 characters, or stored in three 16-bit words for BRF files generated by language compilers like FORTRAN. In the latter case the maximum symbol length is 7 characters, even though technically there is room for 8 characters inside 3 words (16*3 = 48, and 48/6 = 8).

	The 6-bit characters are always packed and right-aligned so that the last character terminates at the end of the last word. For strings shorter than the maximum length the leading 6-bit field(s) are encoded as value 0.		The 6-bit characters are always packed and right-aligned so that the last character terminates at the end of the last word. For strings shorter than the maximum length the leading 6-bit field(s) are encoded as value 0.

Tingo: added Glossary category

2014-07-23T21:46:15Z

added Glossary category

← Older revision		Revision as of 21:46, 23 July 2014
Line 43:		Line 43:
	*{{ND-doc\|60.128.04 \| Rev. A, page 2-64a}}		*{{ND-doc\|60.128.04 \| Rev. A, page 2-64a}}
	<!-- Add a category [Category:??]] -->		<!-- Add a category [Category:??]] -->
			[[Category:Glossary]]

TArntsen: Typo: bit endian -> big endian

2010-07-29T16:01:46Z

Typo: bit endian -> big endian

← Older revision		Revision as of 16:01, 29 July 2010
Line 32:		Line 32:
	*In case of a shorter string, 'B5DE', the encoding would be:		*In case of a shorter string, 'B5DE', the encoding would be:
	0000000000001011 0101000100000101		0000000000001011 0101000100000101
	* 'AB5DE' encoded in three 16-bit ~~bit~~ endian words (long S-group):		* 'AB5DE' encoded in three 16-bit big endian words (long S-group):
	0000000000000000 0000000100001011 0101000100000101		0000000000000000 0000000100001011 0101000100000101

TArntsen: Slightly prettier table

2010-07-29T16:00:34Z

Slightly prettier table

← Older revision		Revision as of 16:00, 29 July 2010
Line 8:		Line 8:
	\| 0x00 \|\| (No character) \|\| -		\| 0x00 \|\| (No character) \|\| -
	\|-		\|-
	\| 0x01..0x1F \|\| "A".."_" \|\| 0x41 - 0x5F		\| 0x01 - 0x1F \|\| "A" - "_" \|\| 0x41 - 0x5F
	\|-		\|-
	\| 0x20..0x3F \|\| ~~" "..~~"?" \|\| 0x20 - 0x3F		\| 0x20 - 0x3F \|\| <space> - "?" \|\| 0x20 - 0x3F
	\|}		\|}

TArntsen: Explain encoding

2010-07-29T15:50:09Z

Explain encoding

New page

Packed six-bit characters is a method used by Norsk Data to create compact character strings. Each character is encoded in 6 bits, and a string is composed of 6-bit characters packed into a continuous bit stream, and stored in a number of 8-bit bytes or 16-bit words, right-aligned so that the least significant bit of the last character is the least significant (or rightmost) bit of the last byte or word (words are big endian on ND).

The character encoding is as follows:

{| class="wikitable"
! 6-bit value !! Character !! 7-bit ASCII value
|-
| 0x00 || (No character) || -
|-
| 0x01..0x1F || "A".."_" || 0x41 - 0x5F
|-
| 0x20..0x3F || " ".."?" || 0x20 - 0x3F
|}

To convert a 6-bit character to ASCII, add 0x40 to the value if the 6-bit value is above 0 but below 0x20, otherwise use as ASCII (or ignore if value is zero).

=== S-group ===
The [[File_Formats#BRF|BRF]] format, i.e. compiled, non-linked ND-100 object code, stores symbols in packed 6-bit characters. These symbols are called S-groups. The symbol may be stored in two 16-bit words for BRF files generated by [[MAC]] and [[NPL]], and contain up to 5 characters, or stored in three 16-bit words for BRF files generated by language compilers like FORTRAN. In the latter case the maximum symbol length is 7 characters, even though technically there is room for 8 characters inside 3 words (16*3 = 48, and 48/6 = 8).

The 6-bit characters are always packed and right-aligned so that the last character terminates at the end of the last word. For strings shorter than the maximum length the leading 6-bit field(s) are encoded as value 0.

==== Example ====

*String 'AB5DE' encoded as short S-group (2 words) or long S-group (3 words)
*The ASCII values are 0x41 0x42 0x35 0x44 0x45
*The 6-bit values are 0x01 0x02 0x35 0x04 0x05 (ref. translation table above).
*Translated to binary:
000001 000010 110101 000100 000101
*Encoded in two 16-bit big endian words (short S-group):
0000000100001011 0101000100000101
*Note how the two first bits are always padding and not part of the first character.
*In case of a shorter string, 'B5DE', the encoding would be:
0000000000001011 0101000100000101
* 'AB5DE' encoded in three 16-bit bit endian words (long S-group):
0000000000000000 0000000100001011 0101000100000101

=== Other uses ===

The [[Monitor call|monitor call]] [[GRTNA]]/[[GetRTName]], mon 152, returns the name of the RT program encoded as packed 6-bit characters in the three 16-bit registers T, A, D.

== References ==
*{{ND-doc|60.196.01|Chapter 6}}
*{{ND-doc|60.128.04 | Rev. A, page 2-64a}}