STRING COMPARISONS
As illustrated in Chapter 4, the string instructions are very powerful because they allow the programmer to manipulate large blocks of data with relative ease. Block data manipulation occurs with the string instructions MOVS, LODS, STOS, INS, and OUTS. In this section, additional string instructions that allow a section of memory to be tested against a constant or against another section of memory are discussed. To accomplish these tasks, use the SCAS (string scan) or CMPS (string compare) instructions.
SCAS
The SCAS (string scan instruction) compares the AL register with a byte block of memory, the AX register with a word block of memory, or the EAX register (80386-Core2) with a double- word block of memory. The SCAS instruction subtracts memory from AL, AX, or EAX without affecting either the register or the memory location. The opcode used for byte comparison is SCASB, the opcode used for the word comparison is SCASW, and the opcode used for a doubleword comparison is SCASD. In all cases, the contents of the extra segment memory location addressed by DI is compared with AL, AX, or EAX. Recall that this default segment (ES) can- not be changed with a segment override prefix.
Like the other string instructions, SCAS instructions use the direction flag (D) to select either auto-increment or auto-decrement operation for DI. They also repeat if prefixed by a conditional repeat prefix.
Suppose that a section of memory is 100 bytes long and begins at location BLOCK. This section of memory must be tested to see whether any location contains 00H. The program in Example 5–33 shows how to search this part of memory for 00H using the SCASB instruction. In this example, the SCASB instruction has an REPNE (repeat while not equal) prefix. The REPNE prefix causes the SCASB instruction to repeat until either the CX register reaches 0, or until an equal condition exists as the outcome of the SCASB instruction’s comparison. Another conditional repeat prefix is REPE (repeat while equal). With either repeat prefix, the contents of CX decrements without affecting the flag bits. The SCASB instruction and the comparison it makes change the flags.
Suppose that you must develop a program that skips ASCII-coded spaces in a memory array. (This task appears in the procedure listed in Example 5–34.) This procedure assumes that the DI register already addresses the ASCII-coded character string and that the length of the string is 256 bytes or fewer. Because this program is to skip spaces (20H), the REPE prefix is used with a SCASB instruction. The SCASB instruction repeats the comparison, searching for a 20H, as long as an equal condition exists.
The CMPS (compare strings instruction) always compares two sections of memory data as bytes (CMPSB), words (CMPSW), or doublewords (CMPSD). Note that only the 80386 through Core2 can use doublewords. In the Pentium 4 or Core2 operated in 64-bit mode, a CMPSQ instruction uses quadwords. The contents of the data segment memory location addressed by SI are compared with the contents of the extra segment memory location addressed by DI. The CMPS instruction increments or decrements both SI and DI. The CMPS instruction is normally used with either the REPE or REPNE prefix. Alternates to these prefixes are REPZ (repeat while zero) and REPNZ (repeat while not zero), but usually the REPE or REPNE prefixes are used in programming.
Example 5–35 illustrates a short procedure that compares two sections of memory searching for a match. The CMPSB instruction is prefixed with REPE. This causes the search to continue as long as an equal condition exists. When the CX register becomes 0 or an unequal condition exists, the CMPSB instruction stops execution. After the CMPSB instruction ends, the CX register is 0 or the flags indicate an equal condition when the two strings match. If CX is not 0 or the flags indicate a not-equal condition, the strings do not match.
