One big advantage of assembly language over HLLs is that assembly language does not limit the size of integers. For example the C programming language defines a maximum of three different integer sizes: short int int and long int. On the PC these are often 16 or 32 bit integers. Although the 80x86 machine instructions limit you to processing eight sixteen or thirty-two bit integers with a single instruction you can always use more than one instruction to process integers of any size you desire. If you want 256 bit integer values no problem. The following sections describe how extended various arithmetic and logical operations from 16 or 32 bits to as many bits as you please.

9.3.1 Multiprecision Addition Operations

The 80x86 add instruction adds two 8 16 or 32 bit numbers[1]. After the execution of the add instruction the 80x86 carry flag is set if there is an overflow out of the H.O. bit of the sum. You can use this information to do multiprecision addition operations. Consider the way you manually perform a multidigit (multiprecision) addition operation:

Step 1: Add the least significant digits together:

         289                             289
+456      produces              +456
----                            ----
5 with carry 1.

Step 2: Add the next significant digits plus the carry:

          1 (previous carry)
289                             289
+456            produces        +456
----                            ----
5                              45 with carry 1.

Step 3: Add the most significant digits plus the carry:

                                         1 (previous carry)
289                             289
+456            produces        +456
----                            ----
45                             745

The 80x86 handles extended precision arithmetic in an identical fashion except instead of adding the numbers a digit at a time it adds them a byte or a word at a time. Consider the three-word (48 bit) addition operation shown below:

The add instruction adds the L.O. words together. The adc (add with carry) instruction adds all other word pairs together. The adc instruction adds two operands plus the carry flag together producing a word value and (possibly) a carry.

For example suppose that you have two thirty-two bit values you wish to add together defined as follows:

X               dword   ?
Y               dword   ?

Suppose also that you want to store the sum in a third variable Z that is likewise defined with the dword directive. The following 80x86 code will accomplish this task:

                mov     ax
word ptr X
add     ax
word ptr Y
mov     word ptr Z
ax
mov     ax
word ptr X+2
adc     ax
word ptr Y+2
mov     word ptr Z+2
ax

Remember these variables are declared with the dword directive. Therefore the assembler will not accept an instruction of the form mov ax X because this instruction would attempt to load a 32 bit value into a 16 bit register. Therefore this code uses the word ptr coercion operator to coerce symbols X Y and Z to 16 bits. The first three instructions add the L.O. words of X and Y together and store the result at the L.O. word of Z. The last three instructions add the H.O. words of X and Y together along with the carry out of the L.O. word and store the result in the H.O. word of Z. Remember address expressions of the form "X+2" access the H.O. word of a 32 bit entity. This is due to the fact that the 80x86 address space addresses bytes and it takes two consecutive bytes to form a word.

Of course if you have an 80386 or later processor you needn't go through all this just to add two 32 bit values together since the 80386 directly supports 32 bit operations. However if you wanted to add two 64 bit integers together on the 80386 you would still need to use this technique.

You can extend this to any number of bits by using the adc instruction to add in the higher order words in the values. For example to add together two 128 bit values you could use code that looks something like the following:

BigVal1         dword   0
0
0
0         ;Four double words in 128 bits!
BigVal2         dword   0
0
0
0
BigVal3         dword   0
0
0
0
.
.
.
mov     eax
BigVal1    ;No need for dword ptr operator since
add     eax
BigVal2    ; these are dword variables.
mov     BigVal3
eax

mov     eax
BigVal1+4  ;Add in the values from the L.O.
adc     eax
BigVal2+4  ; entity to the H.O. entity using
mov     BigVal3+4
eax  ; the ADC instruction.

mov     eax
BigVal1+8
adc     eax
BigVal2+8
mov     BigVal3+8
eax

mov     eax
BigVal1+12
adc     eax
BigVal2+12
mov     BigVal3+12
eax

9.3.2 Multiprecision Subtraction Operations

Like addition the 80x86 performs multi-byte subtraction the same way you would manually except it subtracts whole bytes words or double words at a time rather than decimal digits. The mechanism is similar to that for the add operation You use the sub instruction on the L.O. byte/word/double word and the sbb instruction on the high order values. The following example demonstrates a 32 bit subtraction using the 16 bit registers on the 8086:

var1            dword   ?
var2            dword   ?
diff            dword   ?

mov     ax
word ptr var1
sub     ax
word ptr var2
mov     word ptr diff
ax
mov     ax
word ptr var1+2
sbb     ax
word ptr var2+2
mov     word ptr diff+2
ax

The following example demonstrates a 128-bit subtraction using the 80386 32 bit register set:

BigVal1         dword   0
0
0
0         ;Four double words in 128 bits!
BigVal2         dword   0
0
0
0
BigVal3         dword   0
0
0
0
.
.
.
mov     eax
BigVal1    ;No need for dword ptr operator since
sub     eax
BigVal2    ; these are dword variables.
mov     BigVal3
eax

mov     eax
BigVal1+4  ;Subtract the values from the L.O.
sbb     eax
BigVal2+4  ; entity to the H.O. entity using
mov     BigVal3+4
eax  ; the SUB and SBB instructions.

mov     eax
BigVal1+8
sbb     eax
BigVal2+8
mov     BigVal3+8
eax

mov     eax
BigVal1+12
sbb     eax
BigVal2+12
mov     BigVal3+12
eax

9.3.3 Extended Precision Comparisons

Unfortunately there isn't a "compare with borrow" instruction that can be used to perform extended precision comparisons. Since the cmp and sub instructions perform the same operation at least as far as the flags are concerned you'd probably guess that you could use the sbb instruction to synthesize an extended precision comparison; however you'd only be partly right. There is however a better way.

Consider the two unsigned values 2157h and 1293h. The L.O. bytes of these two values do not affect the outcome of the comparison. Simply comparing 21h with 12h tells us that the first value is greater than the second. In fact the only time you ever need to look at both bytes of these values is if the H.O. bytes are equal. In all other cases comparing the H.O. bytes tells you everything you need to know about the values. Of course this is true for any number of bytes not just two. The following code compares two signed 64 bit integers on an 80386 or later processor:

; This sequence transfers control to location "IsGreater" if
; QwordValue > QwordValue2. It transfers control to "IsLess" if
; QwordValue < QwordValue2. It falls though to the instruction
; following this sequence if QwordValue = QwordValue2. To test for
; inequality
change the "IsGreater" and "IsLess" operands to "NotEqual"
; in this code.

mov     eax
dword ptr QWordValue+4                             ;Get H.O. dword
cmp     eax
dword ptr QWordValue2+4
jg      IsGreater
jl      IsLess
mov     eax
dword ptr QWordValue
cmp     eax
dword ptr QWordValue2
jg      IsGreater
jl      IsLess

To compare unsigned values simply use the ja and jb instructions in place of jg and jl.

You can easily synthesize any possible comparison from the sequence above the following examples show how to do this. These examples do signed comparisons substitute ja jae jb and jbe for jg jge jl and jle (respectively) to do unsigned comparisons.

QW1             qword   ?
QW2             qword   ?

dp              textequ <dword ptr>

; 64 bit test to see if QW1 < QW2 (signed).
; Control transfers to "IsLess" label if QW1 < QW2. Control falls
; through to the next statement if this is not true.

mov     eax
dp QW1+4           ;Get H.O. dword
cmp     eax
dp QW2+4
jg      NotLess
jl      IsLess
mov     eax
dp QW1             ;Fall through to here if H.O.
cmp     eax
dp QW2             ; dwords are equal.
jl      IsLess
NotLess:

; 64 bit test to see if QW1 <= QW2 (signed).

mov     eax
dp QW1+4           ;Get H.O. dword
cmp     eax
dp QW2+4
jg      NotLessEq
jl      IsLessEq
mov     eax
dp QW1
cmp     eax
dword ptr QW2
jle     IsLessEq
NotLessEQ:

; 64 bit test to see if QW1 >QW2 (signed).

mov     eax
dp QW1+4           ;Get H.O. dword
cmp     eax
dp QW2+4
jg      IsGtr
jl      NotGtr
mov     eax
dp QW1             ;Fall through to here if H.O.
cmp     eax
dp QW2             ; dwords are equal.
jg      IsGtr
NotGtr:

; 64 bit test to see if QW1 >= QW2 (signed).

mov     eax
dp QW1+4           ;Get H.O. dword
cmp     eax
dp QW2+4
jg      IsGtrEq
jl      NotGtrEq
mov     eax
dp QW1
cmp     eax
dword ptr QW2
jge     IsGtrEq
NotGtrEq:

; 64 bit test to see if QW1 = QW2 (signed or unsigned). This code branches
; to the label "IsEqual" if QW1 = QW2. It falls through to the next instruction
; if they are not equal.

mov     eax
dp QW1+4           ;Get H.O. dword
cmp     eax
dp QW2+4
jne     NotEqual
mov     eax
dp QW1
cmp     eax
dword ptr QW2
je      IsEqual
NotEqual:

; 64 bit test to see if QW1 <> QW2 (signed or unsigned). This code branches
; to the label "NotEqual" if QW1 <> QW2. It falls through to the next
; instruction if they are equal.

mov     eax
dp QW1+4           ;Get H.O. dword
cmp     eax
dp QW2+4
jne     NotEqual
mov     eax
dp QW1
cmp     eax
dword ptr QW2
jne     NotEqual

[1] As usual 32 bit arithmetic is available only on the 80386 and later processors.

Chapter Nine: Arithmetic And Logical Operations (Part 3)
27 SEP 1996