Binary coded decimal - Revision history

E5frog at 17:45, 13 June 2022

2022-06-13T17:45:09Z

E5frog at 23:48, 30 June 2018

2018-06-30T23:48:01Z

E5frog at 23:46, 30 June 2018

2018-06-30T23:46:05Z

E5frog at 22:26, 16 November 2012

2012-11-16T22:26:30Z

E5frog: 1 revision

2012-11-16T21:22:46Z

1 revision

Admin at 02:28, 24 November 2004

2004-11-24T02:28:11Z

New page

'''Binary coded decimal''' ('''BCD''') is a method by which 4-bit numbers are represented in digits 0-9, not 0-F. In the [[F8]], BCD numbers can be added together using '''ASD''' or from memory using '''AMD''' (there is, however, no '''AID''' [[opcode]], BCD Add Immediate). In a single byte, two digits can be stored, and the appropriate carry flags set if the number rolls over 99. Each digit is coded as the number + 6, so 0 should be loaded as 6, 1 as 7, 2 as 8, etc. This table shows the encoding for individual digits:

<pre>
Digit | 4-bit Value
-------------------
0 | %0110
1 | %0111
2 | %1000
3 | %1001
4 | %1010
5 | %1011
6 | %1100
7 | %1101
8 | %1110
9 | %1111
</pre>

@@ Line 16: / Line 16: @@
 </pre>
-It is very handy to use BCD when you need to present understandable numbers on screen like a score or similar data for the user/player.
+It is very handy to use BCD when you need to present understandable numbers on screen like a score or similar data for the user/player.<br>
+Here's an example used to add up a [[Snippet:Score|Score]].

@@ Line 19: / Line 19: @@
-Here's a recommended way to use BCD and the ASD instructions, taken from Pac-Man.
+Here's a recommended way to use BCD and the ASD instructions, taken from Pac-Man.<br>
 Note the use of the important lnk instruction before asd:
@@ Line 49: / Line 49: @@
 	lr	A, Ku		; load next byte
 	lnk			; add carry from previous operation
-	asd	S		; decimal add scratchpad
+	asd	S		; decimal add scratchpad (byte 4)
-	lr	D, A		; store result
+	lr	S, A		; store result, don't need to decrease ISAR
 	; done
 </pre>

← Older revision		Revision as of 23:46, 30 June 2018
Line 17:		Line 17:

	It is very handy to use BCD when you need to present understandable numbers on screen like a score or similar data for the user/player.		It is very handy to use BCD when you need to present understandable numbers on screen like a score or similar data for the user/player.
		+
		+
		+	Here's a recommended way to use BCD and the ASD instructions, taken from Pac-Man.
		+	Note the use of the important lnk instruction before asd:
		+
		+	<pre>
		+	;---------------------------------------------------------------------------
		+	; Add score
		+	;---------------------------------------------------------------------------
		+	; Adds a 4-byte decimal adjusted number to the score
		+	; stored in r12 (Ku), r13 (Kl), r14 (Qu), r15 (Ql)
		+	; Maximum score with four bytes is 100,000,000-1 (Stored as $FFFFFFFF)
		+
		+	; Score register in this example is 'O'24-'0'27
		+
		+
		+	score.add:
		+	lisu 2 ; ISAR set to O27
		+	lisl 7
		+	lr A, Ql ; load least significant score byte
		+	asd S ; decimal add scratchpad (score_reg)
		+	lr D, A ; save result and then decrease scratchpad
		+	lr A, Qu ; load next byte into A
		+	lnk ; add carry from previous operation
		+	asd S ; decimal add scratchpad (byte 2)
		+	lr D, A ; store result
		+	lr A, Kl ; load next byte
		+	lnk ; add carry from previous operation
		+	asd S ; decimal add scratchpad (byte 3)
		+	lr D, A ; store result
		+	lr A, Ku ; load next byte
		+	lnk ; add carry from previous operation
		+	asd S ; decimal add scratchpad
		+	lr D, A ; store result
		+	; done
		+	</pre>

← Older revision		Revision as of 22:26, 16 November 2012
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	9 \| %1111		9 \| %1111
	</pre>		</pre>
		+
		+	It is very handy to use BCD when you need to present understandable numbers on screen like a score or similar data for the user/player.

← Older revision	Revision as of 21:22, 16 November 2012
(No difference)