32/64-Bit 80x86 Assembly Language Architecture

2017-07-07 02:10:07

Table 15-1: ASCII numerical digit to hex and decimal values
ASCII		1	2	3	4	5	6	7	8	9
Hex	0x30	0x31	0x32	0x33	0x34	0x35	0x36	0x37	0x38	0x39
Decimal	48	49	50	51	52	53	54	55	56	57
BCD		1	2	3	4	5	6	7	8	9
Binary	0000	0001	0010	0011	0100	0101	0110	0111	1000	1001

Converting a BCD value from ASCII to a nibble is as easy as subtracting the hex value of 0x30, '0', or 48 decimal from the ASCII numerical value and get the resulting value with a range of {09}.

byte ASCIItoBCD(char c) { ASSERT(('0' <= c) && (c <= '9')); return (byte)(c - '0'); }

No 64-bit

The general-purpose BCD instructions are not available in 64-bit mode.

When the 8086 processor was first manufactured the FPU was a separate optional chip (8087). There was a need for some BCD operations similar to other processors and so it was incorporated into the CPU. The 8087 had some BCD support as well. When the 64-bit processor was developed, it was decided that BCD support was not required anymore as the FPU was an alternative method.

The FPU uses the first nine bytes to support 18 BCD digits. The uppermost bit of the 10 ^th byte indicates the value is negative if set or positive if the bit is clear.

Figure 15-1: Ten-byte BCD data storage. MSB in far left byte (byte #9) is the sign bit and the rightmost eight bytes (#8...0) contain the BCD value pairs. The 18 ^th BCD digit resides in the upper nibble of byte #8 and the 1 ^st BCD digit resides in the lower nibble of byte #0.

Setting the upper nibble of a byte is merely the shifting left of a BCD digit by four bits, then logical ORing (or suming) the lower nibble.

byte BCDtoByte(byte lo, byte hi) { return (hi << 4) lo; }

DAA Decimal Adjust AL (After) Addition

Mnemonic

PII

3D!

3Mx+

SSE

SSE2

A64

SSE3

E64T

DAA

DAA Decimal Adjust AL (After) Addition

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