DAS (SPC700): Difference between revisions

Latest revision as of 08:19, 23 September 2024


Basic Info
Addressing Mode	Opcode	Length	Speed
Accumulator	BE	1 byte	3 cycles


Flags Affected
N	V	P	B	H	I	Z	C
N	.	.	.	.	.	Z	C

DAS (Decimal Adjust for Subtraction) is an SPC700 instruction that BCD-corrects a difference:

If either the carry flag is clear or the accumulator exceeds 0x99, subtract 0x60 from the accumulator and clear the carry flag. This corrects the tens column (but perhaps not completely).
Then, if either the half-carry flag is clear or the low nibble of the accumulator exceeds 0x09, subtract 0x06 from the accumulator. This corrects the ones column and possibly decrements the tens column by one. Subtracting six effectively maps $F to 9, $E to 8, etc. These hex digits $A to $F can appear in the difference when a borrow occurs.

Before DAS, the value in the accumulator is assumed to be encoded in binary/hexadecimal (with any value in the range $00 to $FF) because that is what the SBC command can output. But the inputs to SBC are supposed to be in BCD. After DAS, the value in the accumulator will be encoded in BCD once again where both nybbles are in the range 0-9. The SPC700 has no decimal mode, so that is why this command is important when working with decimal data.

Syntax

DAS
DAS A

External Links

Official Super Nintendo development manual on DAS: Table C-14 in Appendix C-8 of Book I
page 3-8-2 of Book I, lbid
ares source code, SPC700::instructionDecimalAdjustSub
anomie: https://github.com/yupferris/TasmShiz/blob/8fabc9764c33a7ae2520a76d80ed7220bb939f12/spc700.txt#L410

@@ Line 35: / Line 35: @@
 |}
-'''DAS''' (Decimal Adjust for Subtraction) is an [[SPC700]] instruction that corrects a [[BCD]] difference:
+'''DAS''' (Decimal Adjust for Subtraction) is an [[SPC700]] instruction that [[BCD]]-corrects a difference:
-# If either the [[carry flag]] is clear or the accumulator exceeds 0x99, subtract 0x60 from the accumulator and clear the carry flag.  This corrects the tens column.
+# If either the [[carry flag]] is clear or the accumulator exceeds 0x99, subtract 0x60 from the accumulator and clear the carry flag.  This corrects the tens column (but perhaps not completely).
-# Then, if either the [[half-carry flag]] is clear or the low nibble of the accumulator exceeds 0x09, subtract 0x06 from the accumulator.  This corrects the ones column and possibly decrements the tens column by one.
+# Then, if either the [[half-carry flag]] is clear or the low nibble of the accumulator exceeds 0x09, subtract 0x06 from the accumulator.  This corrects the ones column and possibly decrements the tens column by one.  Subtracting six effectively maps $F to 9, $E to 8, etc.  These hex digits $A to $F can appear in the difference when a borrow occurs.
-Before DAS, the value in the accumulator is assumed to be encoded in binary/hexadecimal because that is what the [[SBC (SPC700)|SBC]] command outputs.  Afterwards, the value in the accumulator will be encoded in BCD.  The SPC700 has no [[decimal mode]], so that is why this command is important when working with decimal data.
+Before DAS, the value in the accumulator is assumed to be encoded in binary/hexadecimal (with any value in the range $00 to $FF) because that is what the [[SBC (SPC700)|SBC]] command can output.  But the inputs to SBC are supposed to be in BCD.  After DAS, the value in the accumulator will be encoded in BCD once again where both nybbles are in the range 0-9.  The SPC700 has no [[decimal mode]], so that is why this command is important when working with decimal data.
 ==== Syntax ====
@@ Line 49: / Line 49: @@
 === See Also ===
 * [[DAA]]
+* [[SED]]
+* [[CLD]]
 === External Links ===