/* personal notes of renzo diomedi */

~ 00001101 ~

Shift Instructions Multiplying and dividing are two of the most time-consuming operations on the processor. However, there are some tricks that you can use to help speed things up. The shift instructions provide a quick and easy way to perform multiplication and division based on powers of 2. This section describes the methods used to perform multiplication and division using the shift instructions.

Shifting utilizes a feature of binary arithmetic that makes multiplying and dividing by powers of 2 simple. You are probably familiar with the decimal world, and how shifting a decimal value into another decimal place automatically multiplies or divides it by a power of 10 (for example, moving the value 2 over one decimal place to 20 is the same as multiplying it by 10).

The same principle applies to binary numbers, except using powers of 2. Shifting a binary number to the left one space multiplies it by 2, two spaces multiplies it by 4, three spaces multiplies it by 8, and so on. Shifting bits in a data element is much quicker than performing binary multiplication and can be used to increase the performance of mathematically intensive programs.

Multiply by shifting

To multiply integers by a power of 2, you must shift the value to the left. Two instructions can be used to left shift integer values, SAL (shift arithmetic left) and SHL (shift logical left). Both of these instructions perform the same operation, and are interchangeable. They have three different formats:

sal destination

sal %cl, destination

sal shifter, destination

The first format shifts the destination value left one position, which is the equivalent of multiplying the value by 2.
The second format shifts the destination value left by the number of times specified in the CL register.
The final version shifts the destination value left the number of times indicated by the shifter value.

In all formats, the destination operand can be an 8-, 16-, or 32-bit register or value in memory. As always, the GNU assembler requires the usual one-character letter appended to the mnemonic to indicate the size of the destination value.
The shift left instructions can be performed on both signed and unsigned integers. The bits emptied by the shift are filled with zeroes. Any bits that are shifted out of the data size are first placed in the carry flag, and then dropped in the next shift. Thus, if a value contains a 1 value in the most significant bit, and is shifted left twice, the most significant bit will be dropped from the carry flag.



salx.exe # windows environment

Breakpoint 1, ?? () at users\rnz\desktop\salx.s:8
8 nop
(gdb) s
9 movl $10, %ebx
(gdb) s
10 sall %ebx
(gdb) s
11 movb $2, %cl
(gdb) s
12 sall %cl, %ebx
(gdb) s
13 sall $2, %ebx
(gdb) s
14 sall value1
(gdb) s
15 sall $2, value1
(gdb) s
16 movl $1, %eax
(gdb) info reg
eax 0x60ffcc 6356940
ecx 0x401002 4198402
edx 0x401000 4198400
ebx 0x140 320
esp 0x60ff74 0x60ff74
ebp 0x60ff80 0x60ff80
esi 0x401000 4198400
edi 0x401000 4198400
eip 0x40101c 0x40101c
eflags 0x212 [ AF IF ]
cs 0x23 35
ss 0x2b 43
ds 0x2b 43
es 0x2b 43
fs 0x53 83
gs 0x2b 43
(gdb) x &value1
0x402000 : 0x000000c8
(gdb) x/d &value1
0x402000 : 200

To follow along, the decimal value 10 is loaded into the EBX register.
The first SAL instruction shifts it one place (multiplying by 2, making 20).
The second SAL instruction shifts it two places (multiplying by four, making 80),
and the third SAL instruction shifts it two more places (multiplying by four, making 320).

The value in the value1 location (25)
is shifted one place (making it 50),
and then shifted two more places (making 200).

Dividing by shifting

Dividing by shifting involves shifting the binary value to the right. However, as you shift an integer value to the right, you must pay attention to the sign of the integer.
For unsigned integers, the bits to the left that are emptied can be filled with zeroes without any problems.
with signed integers, a negative number will be adversely affected by zero-filling the leading bits.
To solve this problem there are two right-shift instructions. The SHR instruction clears the bits emptied by the shift, which makes it useful only for shifting unsigned integers. The SAR instruction either clears or sets the bits emptied by the shift, depending on the sign bit of the integer. For negative numbers, the bits are set to 1, but for positive numbers, they are cleared to zero.

As with the left-shift instructions, the right-shift instructions shift bits out of the data element. Any bits shifted out of the data element (the least significant bits) are first moved to the carry flag, and then shifted out (lost).

Rotating bits

Close relatives to the shift instructions are the rotate instructions. The rotate instructions perform just like the shift instructions, except the overflow bits are pushed back into the other end of the value instead of being dropped. For example, a left rotate of a byte value takes the value in bit position 7 and places it in bit position 0, with each of the other bit positions shifted one place left. The following table shows the various rotate instructions that can be used.

The last two instructions use the carry flag as an additional bit position, to allow 9-bit shifting. The formats of the rotate instructions are the same as the shift instructions, providing three options for their use:
❑ A single operand that is shifted once in the indicated direction
❑ Two operands: The %cl register to indicate the number of times to rotate and the destination operand
❑ Two operands: An immediate value to indicate the number of times to rotate and the destination operand

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