Figure 2-18(a) shows an instruction that uses the relative based indexed addressing mode to add a memory location with a register. From D=0 it is seen that the sum is put in the memory location and W=1 indicates a 16-bit addition. The effective address is found by adding the contents of BX and DI to the 16-bit displacement, which is 2345. If (BX) = 0892 and (DI) = 59A3, then

Figure 2-18(b) shows a similar instruction except that the source operand is immediate. In this instruction S=1 and W=1, which indicate that the 8-bit immediate operand is sign extended to FF97 before it is added. An equivalent instruction could be constructed of 6 bytes by letting S:W = 01 and by including a 16-bit immediate operand containing 97FF.

Figure 2-18 Two examples of the ADD instruction using the relative based indexed addressing mode.

(a) Register to memory addition

(b) Immediate to memory addition

The long and short forms of an instruction for adding an immediate operand to the AX register are given in Fig. 2-19. Because S:W = 01 there are 2 bytes of data in the instruction. In the long form AX is explicitly designated by the R/M field and in the short form AX is implied by the op code. Short forms will be discusses more thoroughly in Sec. 3-12.

Figure 2-19 Two forms for adding an immediate operand to the AX register.

(a) Long form

(b) Short form

Figure 2-20 Sequence of instructions in memory.

Address		Contents
00200		02		First instruction
00201		CF
00202		01		Second instruction
00203		91
00204		45
00205		23
00206		83		Third instruction
00207		81
00208		45
00209		23
0020A		97
0020B		05		Fourth instruction
0020C		23
0020D		01
0020E

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