15.1.4 - The MOVS Instruction

15.1.5 - The CMPS Instruction

15.1.6 - The SCAS Instruction

15.1.7 - The STOS Instruction

15.1.8 - The LODS Instruction

15.1.9 - Building Complex String Functions from LODS and STOS

15.1.10 - Prefixes and the String Instructions

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15.1.4 The MOVS Instruction

        {REP} MOVSB
        {REP} MOVSW
        {REP} MOVSD                     ;Available only on 80386+
        {REP} MOVS      Dest, Source

The movsb (move string, bytes) instruction fetches the byte at address ds:si, stores it at address es:di, and then increments or decrements the si and di registers by one. If the rep prefix is present, the CPU checks cx to see if it contains zero. If not, then it moves the byte from ds:si to es:di and decrements the cx register. This process repeats until cx becomes zero.

The movsw (move string, words) instruction fetches the word at address ds:si, stores it at address es:di, and then increments or decrements si and di by two. If there is a rep prefix, then the CPU repeats this procedure as many times as specified in cx.

The movsd instruction operates in a similar fashion on double words. Incrementing or decrementing si and di by four for each data movement.

MASM automatically figures out the size of the movs instruction by looking at the size of the operands specified. If you've defined the two operands with the byte (or comparable) directive, then MASM will emit a movsb instruction. If you've declared the two labels via word (or comparable), MASM will generate a movws instruction. If you've declared the two labels with dword, MASM emits a movsd instruction. The assembler will also check the segments of the two operands to ensure they match the current assumptions (via the assume directive) about the es and ds registers. You should always use the movsb, movsw, and movsd forms and forget about the movs form.

Although, in theory, the movs form appears to be an elegant way to handle the move string instruction, in practice it creates more trouble than it's worth. Furthermore, this form of the move string instruction implies that movs has explicit operands, when, in fact, the si and di registers implicitly specify the operands. For this reason, we'll always use the movsb, movsw, or movsd instructions. When used with the rep prefix, the movsb instruction will move the number of bytes specified in the cx register. The following code segment copies 384 bytes from String1 to String2:

                cld
                lea     si, String1
                lea     di, String2
                mov     cx, 384
        rep     movsb
                 .
                 .
                 .
String1         byte    384 dup (?)
String2         byte    384 dup (?)

This code, of course, assumes that String1 and String2 are in the same segment and both the ds and es registers point at this segment. If you substitute movws for movsb, then the code above will move 384 words (768 bytes) rather than 384 bytes:

                cld
                lea     si, String1
                lea     di, String2
                mov     cx, 384
        rep     movsw
                 .
                 .
                 .
String1         word    384 dup (?)
String2         word    384 dup (?)

Remember, the cx register contains the element count, not the byte count. When using the movsw instruction, the CPU moves the number of words specified in the cx register.

If you've set the direction flag before executing a movsb/movsw/movsd instruction, the CPU decrements the si and di registers after moving each string element. This means that the si and di registers must point at the end of their respective strings before issuing a movsb, movsw, or movsd instruction. For example,

                std
                lea     si, String1+383
                lea     di, String2+383
                mov     cx, 384
        rep     movsb
                 .
                 .
                 .
String1         byte    384 dup (?)
String2         byte    384 dup (?)

Although there are times when processing a string from tail to head is useful (see the cmps description in the next section), generally you'll process strings in the forward direction since it's more straightforward to do so. There is one class of string operations where being able to process strings in both directions is absolutely mandatory: processing strings when the source and destination blocks overlap. Consider what happens in the following code:

                cld
                lea     si, String1
                lea     di, String2
                mov     cx, 384
        rep     movsb
                 .
                 .
                 .
String1         byte    ?
String2         byte    384 dup (?)

This sequence of instructions treats String1 and String2 as a pair of 384 byte strings. However, the last 383 bytes in the String1 array overlap the first 383 bytes in the String2 array. Let's trace the operation of this code byte by byte.

When the CPU executes the movsb instruction, it copies the byte at ds:si (String1) to the byte pointed at by es:di (String2). Then it increments si and di, decrements cx by one, and repeats this process. Now the si register points at String1+1 (which is the address of String2) and the di register points at String2+1. The movsb instruction copies the byte pointed at by si to the byte pointed at by di. However, this is the byte originally copied from location String1. So the movsb instruction copies the value originally in location String1 to both locations String2 and String2+1. Again, the CPU increments si and di, decrements cx, and repeats this operation. Now the movsb instruction copies the byte from location String1+2 (String2+1) to location String2+2. But once again, this is the value that originally appeared in location String1. Each repetition of the loop copies the next element in String1 to the next available location in the String2 array. Pictorially, it looks something like:



The end result is that X gets replicated throughout the string. The move instruction copies the source operand into the memory location which will become the source operand for the very next move operation, which causes the replication.

If you really want to move one array into another when they overlap, you should move each element of the source string to the destination string starting at the end of the two strings as shown below:



Setting the direction flag and pointing si and di at the end of the strings will allow you to (correctly) move one string to another when the two strings overlap and the source string begins at a lower address than the destination string. If the two strings overlap and the source string begins at a higher address than the destination string, then clear the direction flag and point si and di at the beginning of the two strings.

If the two strings do not overlap, then you can use either technique to move the strings around in memory. Generally, operating with the direction flag clear is the easiest, so that makes the most sense in this case.

You shouldn't use the movs instruction to fill an array with a single byte, word, or double word value. Another string instruction, stos, is much better suited for this purpose. However, for arrays whose elements are larger than four bytes, you can use the movs instruction to initialize the entire array to the content of the first element. See the questions for additional information.

15.1.5 The CMPS Instruction

        {REPE}  CMPSB
        {REPE}  CMPSW
        {REPE}  CMPSD                   ;Available only on 80386+
        {REPE}  CMPS    dest, source
        {REPNE} CMPSB
        {REPNE} CMPSW 
        {REPNE} CMPSD                   ;Available only on 80386+
        {REPNE} CMPS    dest, source

Like the movs instruction, the operands present in the operand field of the cmps instruction determine the size of the operands. You specify the actual operand addresses in the si and di registers.

Without a repeat prefix, the cmps instruction subtracts the value at location es:di from the value at ds:si and updates the flags. Other than updating the flags, the CPU doesn't use the difference produced by this subtraction. After comparing the two locations, cmps increments or decrements the si and di registers by one, two, or four (for cmpsb/cmpsw/cmpsd, respectively). Cmps increments the si and di registers if the direction flag is clear and decrements them otherwise.

Of course, you will not tap the real power of the cmps instruction using it to compare single bytes or words in memory. This instruction shines when you use it to compare whole strings. With cmps, you can compare consecutive elements in a string until you find a match or until consecutive elements do not match.

To compare two strings to see if they are equal or not equal, you must compare corresponding elements in a string until they don't match. Consider the following strings:

"String1"

"String1"

The only way to determine that these two strings are equal is to compare each character in the first string to the corresponding character in the second. After all, the second string could have been "String2" which definitely is not equal to "String1". Of course, once you encounter a character in the destination string which doesn't equal the corresponding character in the source string, the comparison can stop. You needn't compare any other characters in the two strings.

The repe prefix accomplishes this operation. It will compare successive elements in a string as long as they are equal and cx is greater than zero. We could compare the two strings above using the following 80x86 assembly language code:

; Assume both strings are in the same segment and ES and DS
; both point at this segment.

                cld
                lea     si, AdrsString1
                lea     di, AdrsString2
                mov     cx, 7
        repe    cmpsb

After the execution of the cmpsb instruction, you can test the flags using the standard conditional jump instructions. This lets you check for equality, inequality, less than, greater than, etc.

Character strings are usually compared using lexicographical ordering. In lexicographical ordering, the least significant element of a string carries the most weight. This is in direct contrast to standard integer comparisons where the most significant portion of the number carries the most weight. Furthermore, the length of a string affects the comparison only if the two strings are identical up to the length of the shorter string. For example, "Zebra" is less than "Zebras", because it is the shorter of the two strings, however, "Zebra" is greater than "AAAAAAAAAAH!" even though it is shorter. Lexicographical comparisons compare corresponding elements until encountering a character which doesn't match, or until encountering the end of the shorter string. If a pair of corresponding characters do not match, then this algorithm compares the two strings based on that single character. If the two strings match up to the length of the shorter string, we must compare their length. The two strings are equal if and only if their lengths are equal and each corresponding pair of characters in the two strings is identical. Lexicographical ordering is the standard alphabetical ordering you've grown up with.

For character strings, use the cmps instruction in the following manner:

• The direction flag must be cleared before comparing the strings.
• 
  
• Use the cmpsb instruction to compare the strings on a byte by byte basis. Even if the strings contain an even number of characters, you cannot use the cmpsw instruction. It does not compare strings in lexicographical order.
• 
  
• The cx register must be loaded with the length of the smaller string.
• 
  
• Use the repe prefix.
• 
  
• The ds:si and es:di registers must point at the very first character in the two strings you want to compare.

After the execution of the cmps instruction, if the two strings were equal, their lengths must be compared in order to finish the comparison. The following code compares a couple of character strings:

                lea     si, source
                lea     di, dest
                mov     cx, lengthSource
                mov     ax, lengthDest
                cmp     cx, ax
                ja      NoSwap
                xchg    ax, cx
NoSwap:         repe    cmpsb
                jne     NotEqual
                mov     ax, lengthSource
                cmp     ax, lengthDest
NotEqual:

If you're using bytes to hold the string lengths, you should adjust this code appropriately.

You can also use the cmps instruction to compare multi-word integer values (that is, extended precision integer values). Because of the amount of setup required for a string comparison, this isn't practical for integer values less than three or four words in length, but for large integer values, it's an excellent way to compare such values. Unlike character strings, we cannot compare integer strings using a lexicographical ordering. When comparing strings, we compare the characters from the least significant byte to the most significant byte. When comparing integers, we must compare the values from the most significant byte (or word/double word) down to the least significant byte, word or double word. So, to compare two eight-word (128-bit) integer values, use the following code on the 80286:

                std
                lea     si, SourceInteger+14
                lea     di, DestInteger+14
                mov     cx, 8
        repe    cmpsw

This code compares the integers from their most significant word down to the least significant word. The cmpsw instruction finishes when the two values are unequal or upon decrementing cx to zero (implying that the two values are equal). Once again, the flags provide the result of the comparison.

The repne prefix will instruct the cmps instruction to compare successive string elements as long as they do not match. The 80x86 flags are of little use after the execution of this instruction. Either the cx register is zero (in which case the two strings are totally different), or it contains the number of elements compared in the two strings until a match. While this form of the cmps instruction isn't particularly useful for comparing strings, it is useful for locating the first pair of matching items in a couple of byte or word arrays. In general, though, you'll rarely use the repne prefix with cmps.

One last thing to keep in mind with using the cmps instruction - the value in the cx register determines the number of elements to process, not the number of bytes. Therefore, when using cmpsw, cx specifies the number of words to compare. This, of course, is twice the number of bytes to compare.

15.1.6 The SCAS Instruction

        {REPE}  SCASB
        {REPE}  SCASW
        {REPE}  SCASD           ;Available only on 80386+
        {REPE}  SCAS    dest
        {REPNE} SCASB
        {REPNE} SCASW
        {REPNE} SCASD           ;Available only on 80386+
        {REPNE} SCAS    dest

Like the cmps and movs instructions, the value in the cx register specifies the number of elements to process, not bytes, when using a repeat prefix.

15.1.7 The STOS Instruction

; Presumably, the ES register already points at the segment
; containing DestString 

                cld
                lea     di, DestString
                mov     cx, 128         ;256 bytes is 128 words.
                xor     ax, ax          ;AX := 0
        rep     stosw

This code writes 128 words rather than 256 bytes because a single stosw operation is faster than two stosb operations. On an 80386 or later this code could have written 64 double words to accomplish the same thing even faster.

The stos instruction takes four forms. They are

        {REP}   STOSB
        {REP}   STOSW
        {REP}   STOSD
        {REP}   STOS    dest

The stosb instruction stores the value in the al register into the specified memory location(s), the stosw instruction stores the ax register into the specified memory location(s) and the stosd instruction stores eax into the specified location(s). The stos instruction is either an stosb, stosw, or stosd instruction depending upon the size of the specified operand.

Keep in mind that the stos instruction is useful only for initializing a byte, word, or dword array to a constant value. If you need to initialize an array to different values, you cannot use the stos instruction. You can use movs in such a situation, see the exercises for additional details.

15.1.8 The LODS Instruction

        {REP}   LODSB
        {REP}   LODSW
        {REP}   LODSD           ;Available only on 80386+
        {REP}   LODS    dest

As mentioned earlier, you'll rarely, if ever, use the rep prefixes with these instructions[3]. The 80x86 increments or decrements si by one, two, or four depending on the direction flag and whether you're using the lodsb, lodsw, or lodsd instruction.

15.1.9 Building Complex String Functions from LODS and STOS

; Presumably, ES and DS have been set up to point at the same
; segment, the one containing the string to convert.

                lea     si, String2Convert
                mov     di, si
                mov     cx, LengthOfString
Convert2Lower:  lodsb                           ;Get next char in str.
                cmp     al, 'A'                 ;Is it upper case?
                jb      NotUpper
                cmp     al, 'Z'
                ja      NotUpper
                or      al, 20h                 ;Convert to lower case. 
NotUpper:       stosb                           ;Store into destination.
                loop    Convert2Lower

Assuming you're willing to waste 256 bytes for a table, this conversion operation can be sped up somewhat using the xlat instruction:

; Presumably, ES and DS have been set up to point at the same
; segment, the one containing the string to be converted.

                cld
                lea     si, String2Convert
                mov     di, si
                mov     cx, LengthOfString
                lea     bx, ConversionTable
Convert2Lower:  lodsb                           ;Get next char in str.
                xlat                            ;Convert as appropriate.
                stosb                           ;Store into destination.
                loop    Convert2Lower

The conversion table, of course, would contain the index into the table at each location except at offsets 41h..5Ah. At these locations the conversion table would contain the values 61h..7Ah (i.e., at indexes 'A'..'Z' the table would contain the codes for 'a'..'z').

Since the lods and stos instructions use the accumulator as an intermediary, you can use any accumulator operation to quickly manipulate string elements.

15.1.10 Prefixes and the String Instructions

15.1.4 - The MOVS Instruction

15.1.5 - The CMPS Instruction

15.1.6 - The SCAS Instruction

15.1.7 - The STOS Instruction

15.1.8 - The LODS Instruction

15.1.9 - Building Complex String Functions from LODS and STOS

15.1.10 - Prefixes and the String Instructions

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