Difference between revisions of "DAA"
From GbdevWiki
(Created page with ' +---+---+---+--------------+--------------+--------------+---------+ | N | H | C | Upper Nybble | Lower Nybble | Number Added | C After | +---+---+---+--------------+---------…') |
|||
| Line 1: | Line 1: | ||
| − | + | If an addition operation is performed between 15 (BCD) and 27 (BCD), simple decimal arithmetic gives this result: | |
| − | + | ||
| − | + | + | 15 |
| − | + | + 27 | |
| − | + | -- | |
| − | + | 42 | |
| − | + | ||
| − | + | But when the binary representations are added in the Accumulator according to standard binary arithmetic. | |
| − | + | ||
| − | + | 0001 0101 | |
| − | + | + 0010 0111 | |
| − | + | --------- | |
| − | + | 0011 1100 = $3C | |
| − | + | ||
| − | + | the sum is ambiguous. The DAA instruction adjusts this result so that the correct BCD representation is obtained: | |
| − | + | 0011 1100 | |
| − | + | + | + 0000 0110 |
| + | 0100 0010 = $42 | ||
== Instructions == | == Instructions == | ||
| Line 25: | Line 26: | ||
Z: 1 if Accumulator is zero after operation; 0 otherwise | Z: 1 if Accumulator is zero after operation; 0 otherwise | ||
N: not affected | N: not affected | ||
| − | H: | + | H: 0 |
| − | C: | + | C: 1 if N is 0 and A is greater than $99, 0 otherwise |
| + | |||
| + | == Pseudo-code == | ||
| + | |||
| + | void DAA(void) | ||
| + | { | ||
| + | if (n) | ||
| + | { | ||
| + | if (c) { reg.a -= 0x60; } | ||
| + | if (h) { reg.a -= 0x06; } | ||
| + | } | ||
| + | else | ||
| + | { | ||
| + | if (c || (reg.a & 0xFF) > 0x99) { reg.a += 0x60; c = 1; } | ||
| + | if (h || (reg.a & 0x0F) > 0x09) { reg.a += 0x06; } | ||
| + | } | ||
| + | |||
| + | z = reg.a == 0; | ||
| + | h = 0; | ||
| + | } | ||
Revision as of 08:46, 25 May 2012
If an addition operation is performed between 15 (BCD) and 27 (BCD), simple decimal arithmetic gives this result:
15 + 27 -- 42
But when the binary representations are added in the Accumulator according to standard binary arithmetic.
0001 0101 + 0010 0111 --------- 0011 1100 = $3C
the sum is ambiguous. The DAA instruction adjusts this result so that the correct BCD representation is obtained:
0011 1100 + 0000 0110 0100 0010 = $42
Instructions
DAA - $27 - 1 Machine Cycle
Flags
Z: 1 if Accumulator is zero after operation; 0 otherwise N: not affected H: 0 C: 1 if N is 0 and A is greater than $99, 0 otherwise
Pseudo-code
void DAA(void)
{
if (n)
{
if (c) { reg.a -= 0x60; }
if (h) { reg.a -= 0x06; }
}
else
{
if (c || (reg.a & 0xFF) > 0x99) { reg.a += 0x60; c = 1; }
if (h || (reg.a & 0x0F) > 0x09) { reg.a += 0x06; }
}
z = reg.a == 0;
h = 0;
}
