IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

OPERAND SUMMARY
"reg" field Bit Assignments:
16-Bit (w=l) a-Bit (w=O) Segment
000 AX 000 AL 00 ES
001 CX 001 CL 01 CS
010 OX 010 OL 10 SS
011 BX 011 BL 11 OS
100 SP 100 AH
101 BP 101 CH
110 SI 110 OH
111 01 111 BH
SECOND INSTRUCTION BYTE SUMMARY
Imod Ixxx Irim I
mod
Displacement
00 OISP = 0*, disp-Iow and disp-high are absent
01 OISP = disp-Iow sign-extended to 16-bits, disp-high is absent
10 OISP = disp-high: disp-Iow
11 rim is treated as a "reg" field
rIm
Operand Address
000 (BX) + (SI) + DISP
001 (BX) + (01) + DISP
010 (BP) + (SI) + OISP
011 (BP) + (01) + OISP
100 (SI) + DISP
101 (01) + OISP
110 (BP)+OISP*
111 (BX) + OISP
OISP follows 2nd byte of instruction (before data if required).
*except if mod = 00 and rim = 110 then EA = disp-high: disp-Iow.
B-3