The Intel 80386, part 12: The stuff you don’t need to know

Raymond Chen

Raymond

There are quite a few extra instructions that are technically legal in user-mode code, but which you won’t see in compiler-generated code because they are simply too weird.

    PUSHAD              ; push all general-purpose registers
    POPAD               ; pop (almost) all general-purpose registers
    PUSHFD              ; push flags register
    POPFD               ; pop flags register
    LAHF                ; AH = flags
    SAHF                ; flags = AH

The PUSHAD (push all doubleword) and POPAD (pop all doubleword) instructions push and pop the eight general-purpose registers onto the stack. This includes the stack pointer register esp! The PUSHAD instruction pushes the esp register onto the stack, and the POPAD instruction pops the value, but doesn’t store it into the esp register. The value that would normally go into the esp register is simply discarded.

The PUSHFD (push flags doubleword) and POPFD (pop flags doubleword) instructions push and pop the flags register to/from the stack. When popped, some flag bits are discarded rather than being stored into the flags register.

The LAHF (load ah from flags) and the SAHF (store ah to flags) instructions transfer the sf,zf, af, pf, and cf flags to and from the ah register.

The next group of instructions are for binary-coded decimal. Packed binary coded decimal (packed BCD) uses a single byte to represent values from 0 to 99, putting the tens digit in the upper nibble and the units digit in the lower nibble. Each subsequent byte represents another power of 100. For example, the decimal number 764 is represented by the byte 0x64 followed by the byte 0x07. In the mnenonic, this is known as “decimal” BCD.

Unpacked binary coded decimal (unpacked BCD) uses a single byte to represent a single digit from 0 to 9. The value simply represents itself. Each subsequent byte represents another power of ten. For example, the decimal number 764 is represented by the bytes 0x04, 0x06 and 0x07. In the mnenonic, this is known as “ASCII” BCD.

    DAA                 ; decimal (packed BCD) adjust after addition
    DAS                 ; decimal (packed BCD) adjust after subtraction
    AAA                 ; ASCII (unpacked BCD) adjust after addition
    AAS                 ; ASCII (unpacked BCD) adjust after subtraction
    AAM                 ; ASCII (unpacked BCD) adjust after multiplication
    AAD                 ; ASCII (unpacked BCD) adjust after division

All of the BCD adjustment instructions are expected to be executed immediately after the corresponding arithmetic operation, and the destination of the arithmetic operation is expected to be the al register. I mean, there’s nothing preventing you from executing the instructions even if you didn’t meet the prerequisites, but the results are not likely to be very useful.

The DAA instruction (decimal adjust after addition) assumes that you added two bytes in packed BCD format, and it converts the result back into packed BCD format, setting the carry flag according to whether the result was 100 or greater. For example, if you added 0x23 and 0x59, the initial result is 0x7C (which is the sum as normal integers), and the the DAA instruction adjusts the value to 0x82, to represent the packed BCD sum.

The DAS (decimal adjust after subtraction) instruction operates similarly.

The AAA (ASCII adjust after addition) assumes that the operation you performed was on an unpacked BCD value. It adjusts the value in the al, and if a carry occured, it increments the ah register. The AAS (ASCII adjust after subtraction) operates similarly.

The AAM (ASCII adjust after multiplication) instruction assumes that the most recent operation was a multiply of two 8-bit values in unpacked BCD format, producing a result in ax.

The AAD (ASCII adjust before division) instruction is unusual in that you execute it before the corresponding instruction. It takes an unpacked BCD two-digit value in ax and prepares it so that the upcoming 16-by-8 division will produce correct decimal values.

The next instructions are for bit-scanning.

    BSF     r, r/m      ; d = index of first set bit in s
    BSR     r, r/m	; d = index of last set bit in s

The BSF (bit scan forward) instruction searches for the least significant set bit in the source value and sets the destination register to the index of that bit. The BSR (bit scan reverse) instruction does the same, but it looks for the most significant set bit. If the source is zero, then the destination is undefined and the zf flag is set.

The next group is the rotation instructions.

    ROL     r/m, CL/i   ; d = d rotate left by s, set flags
    ROR     r/m, CL/i   ; d = d rotate left by s, set flags
    RCL     r/m, CL/i   ; d = d|CF rotate left by s, set flags
    RCR     r/m, CL/i   ; d = d|CF rotate left by s, set flags

The ROL instruction rotates the bits of the destination left (towards higher significance) by the amount specified by the source, which is taken mod 32. The ROR instruction rotates right. The carry flag contains the last bit rotated out, and if the shift amount is the immediate 1, then the overflow flag is set if the sign bit changed. (If the shift amount is not the immediate 1, then the overflow flag is undefined.) The zero, sign, and parity flags are set based on the result.

The RCL and RCR instructions are similar, except that rotation is through an n+1 bit value, where the carry flag is the extra bit.

And then there are the counted loop instructions.

    LOOP    dest        ; decrement ecx, jump if result is nonzero
    LOOPE   dest        ; decrement ecx, jump if result is nonzero
                        ; and ZF is set (alternate opcode: LOOPZ)
    LOOPNE  dest        ; decrement ecx, jump if result is nonzero
                        ; and ZF is clear (alternate opcode: LOOPNZ)
    JECXZ   dest        ; jump if ecx is zero

The counted loop instructions require the loop counter to be stored in the ecx register. The usual pattern is

    MOV     ecx, number_of_iterations
    JECXZ   done        ; no iterations at all
again:
    ... do something ...
    LOOP    again       ; do it number_of_iterations times
done:

You can also make the loop conditional upon the zf flag. The LOOPE (loop while equal) instruction loops provided the result of the most recent flags-setting operation was zero. The LOOPNE (loop while not equal) requires that the most recent flags-setting result be nonzero.

    MOV     ecx, number_of_iterations
    JECXZ   done        ; no iterations at all
again:
    ... do something ...
    CMP     eax, 90
    LOOPZ   again       ; do it number_of_iterations times
                        ; provided eax is 90
done:
    ; loop ends when we have executed all iterations or eax is not 90

And then some random instructions I couldn’t categorize easily.

    XLAT                ; al = byte at ebx+al
    BOUND   r, m        ; check that d is in range [s]..[s+4]
    INTO                ; check if overflow is set

The XLAT instruction treats the value in the al register as an index into in a table of 256 bytes starting at ebx, putting the result back into the al register. My guess, given the opcode name, is that this was for character set translation where the characters in the source and destination character sets are both single-byte. (Think ASCII and EBCDIC.)

The BOUND instruction performs a bounds check of the destination register. The source refers to two 32-bit values in memory, the first being the smallest legal value and the second being the largest legal value. If the destination value is not in range, then interrupt 5 is raised. The values are treated as unsigned because the intended purpose of this instruction is to perform an array bounds check.

The INTO instruction checks whether the overflow bit is set. If so, then it raises interrupt 4.

Finally, there are instructions so weird I won’t even go into them. They are technically legal instructions but are not useful in practice because 32-bit Windows uses a flat address space.

    ARPL    r/m16, r16  ; adjust requested privilege level
    LAR     r32, r/m32  ; load access rights
    LSL     r32, r/m32  ; load selector limit

These instructions operate on selectors, but since there are no interesting selectors in 32-bit Windows (aside from the TEB, which we discussed earlier), these instructions don’t accomplish anything interesting.

Next time, we’ll look at the Windows calling conventions.

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