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GX1 Datasheet(PDF) 216 Page - National Semiconductor (TI) |
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GX1 Datasheet(HTML) 216 Page - National Semiconductor (TI) |
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216 / 247 page  www.national.com 216 Revision 1.0 Instruction Set (Continued) 8.1.4 reg Field The reg field (Table 8-10) determines which general regis- ters are to be used. The selected register is dependent on whether a 16- or 32-bit operation is current and on the status of the w bit. 8.1.4.1 sreg2 Field (ES, CS, SS, DS Register Selection) The sreg2 field (Table 8-11) is a 2-bit field that allows one of the four 286-type segment registers to be specified. 8.1.4.2 sreg3 Field (FS and GS Segment Register Selection) The sreg3 field (Table 8-12) is 3-bit field that is similar to the sreg2 field, but allows use of the FS and GS segment registers. 8.1.5 s-i-b Byte (Scale, Indexing, Base) The s-i-b fields provide scale factor, indexing and a base field for address selection. The ss, index and base fields are described next. 8.1.5.1 ss Field (Scale Selection) The ss field (Table 8-13) specifies the scale factor used in the offset mechanism for address calculation. The scale factor multiplies the index value to provide one of the com- ponents used to calculate the offset address. Table 8-9. General Registers Selected by mod r/m Fields and w Field mod r/m 16-Bit Operation 32-Bit Operation w= 0 w =1 w = 0 w =1 11 000 AL AX AL EAX 11 001 CL CX CL ECX 11 010 DL DX DL EDX 11 011 BL BX BL EBX 11 100 AH SP AH ESP 11 101 CH BP CH EBP 11 110 DH SI DH ESI 11 111 BH DI BH EDI Table 8-10. General Registers Selected by reg Field reg 16-Bit Operation 32-Bit Operation w=0 w = 1 w=0 w = 1 000 AL AX AL EAX 001 CL CX CL ECX 010 DL DX DL EDX 011 BL BX BL EBX 100 AH SP AH ESP 101 CH BP CH EBP 110 DH SI DH ESI 111 BH DI BH EDI Table 8-11. sreg2 Field Encoding sreg2 Field Segment Register Selected 00 ES 01 CS 10 SS 11 DS Table 8-12. sreg3 Field Encoding sreg3 Field Segment Register Selected 000 ES 001 CS 010 SS 011 DS 100 FS 101 GS 110 Undefined 111 Undefined Table 8-13. ss Field Encoding ss Field Scale Factor 00 x1 01 x2 01 x4 11 x8 |
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