Data Movement Instructions A Course in Microprocessor Electrical Engineering Department University 17 Agustus 1945 Jakarta

Load-Effective Address There are several LEA instructions in p Table 4.9 lists the LEA instructions LEA The LEA instruction loads a 16- or 32- bit register with the offset address of the data specified by the operand E.g., LEA AX,NUMB Compare: LEA BX,[DI] & MOV BX,[DI] Understand Example 4.3

Load-Effective Address (cont’d) LDS, LES, LFS, LGS, LSS The LDS, LES, LFS, LGS, LSS instructions load any 16- or 32-bit register with an offset address and load the DS, ES, FS, GS, or SS segment register with a segment address Fig. 4.15 illustrates an example LDS BX,[DI] instruction this instruction transfers the 32-bit number, address-ed by DI in the data segment, into the BX and DS registers Study Example 4.4

String Data Transfers There are 5 string data transfer instructions: LODS, STOS, MOVS, INS, and OUTS The Direction Flag The direction flag (D) -located in the flag register- selects the auto-increment (D=O) or the auto-decrement (D=1) operation for the DI and SI registers during string operations The CLD instruction clears the D flag (D=0), and the STD instruction sets it (D=1)

String Data Transfers (cont’d) DI and SI The DI offset address accesses data in the extra segment for all string instructions that use it The SI offset address accesses data, by default, in the data segment LODS The LODS instruction loads AL, AX, or EAX with data stored at the data segment offset address indexed by the SI register (Table 4.10)

String Data Transfers (cont’d) Fig.4.16 shows the effect of executing the LODSW instruction if the D flag=0, SI=1000H, and DS=1000H STOS The STOS instruction stores AL, AX, or EAX at the extra segment memory location addressed by the DI register Table 4.11 lists all forms of STOS instructions The STOSB (stores a byte), STOSW (stores a word) and STOSD (stores a doubleword)

String Data Transfers (cont’d) In example 4.5 STOSW instruction is used to clear the video text display MOVS MOVS instruction transfers data (either byte, word or doubleword) from one memory loca-tion to another (Table 4.13) MOVS transfers data from the data segment location addressed by SI to the extra segment location addressed by DI Example 4.6 lists a short program that uses MOVS

String Data Transfers (cont’d) INS The INS (input string) instruction transfers data from an I/O device into extra segment memory location addressed by the DI register The I/O address is contained in the DX register See Table 4.14 and Study example 4.7 OUTS The OUTS (output string) transfers data from the data segment location addressed by SI to an I/O device (addressed by DX) -- Table 4.15 & Example 4.8