Difference between revisions of "Binary coded decimal"

From veswiki
Jump to: navigation, search
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:
 
Note the use of the important lnk instruction before asd:
  
Line 49: Line 49:
 
lr A, Ku ; load next byte
 
lr A, Ku ; load next byte
 
lnk ; add carry from previous operation
 
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
 
; done
 
</pre>
 
</pre>

Revision as of 23:48, 30 June 2018

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:

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

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:

;---------------------------------------------------------------------------
; 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 (byte 4)
	lr	S, A		; store result, don't need to decrease ISAR
	; done