CS301: Computer Architecture

Read this article, which gives two examples of instructions set architectures (ISAs). Look over how the different microprocessors address memory. Take note of similarities and differences of format, instructions and type of instructions, and addressing modes between these two as well as between these and the MIPS instructions of the previous sections.

Shift and Rotate Instructions

Logical Shift Instructions

In a logical shift instruction (also referred to as unsigned shift), the bits that slide off the end disappear (except for the last, which goes into the carry flag), and the spaces are always filled with zeros. Logical shifts are best used with unsigned numbers.

shr src, dest GAS Syntax
shr dest, src Intel Syntax

Logical shift dest to the right by src bits.

shl src, dest GAS Syntax
shl dest, src Intel Syntax

Logical shift dest to the left by src bits.

Examples (GAS Syntax):

movw $ff00,%ax # ax=1111.1111.0000.0000 (0xff00, unsigned 65280, signed -256) shrw $3,%ax # ax=0001.1111.1110.0000 (0x1fe0, signed and unsigned 8160) # (logical shifting unsigned numbers right by 3 # is like integer division by 8) shlw $1,%ax # ax=0011.1111.1100.0000 (0x3fc0, signed and unsigned 16320) # (logical shifting unsigned numbers left by 1 # is like multiplication by 2)

Arithmetic Shift Instructions

In an arithmetic shift (also referred to as signed shift), like a logical shift, the bits that slide off the end disappear (except for the last, which goes into the carry flag). But in an arithmetic shift, the spaces are filled in such a way to preserve the sign of the number being slid. For this reason, arithmetic shifts are better suited for signed numbers in two's complement format.

sar src, dest GAS Syntax
sar dest, src Intel Syntax

Arithmetic shift dest to the right by src bits. Spaces are filled with sign bit (to maintain sign of original value), which is the original highest bit.

sal src, dest GAS Syntax
sal dest, src Intel Syntax

Arithmetic shift dest to the left by src bits. The bottom bits do not affect the sign, so the bottom bits are filled with zeros. This instruction is synonymous with SHL.

Examples (GAS Syntax):

movw $ff00,%ax # ax=1111.1111.0000.0000 (0xff00, unsigned 65280, signed -256) salw $2,%ax # ax=1111.1100.0000.0000 (0xfc00, unsigned 64512, signed -1024) # (arithmetic shifting left by 2 is like multiplication by 4 for # negative numbers, but has an impact on positives with most # significant bit set (i.e. set bits shifted out)) sarw $5,%ax # ax=1111.1111.1110.0000 (0xffe0, unsigned 65504, signed -32) # (arithmetic shifting right by 5 is like integer division by 32 # for negative numbers)

Extended Shift Instructions

The names of the double precision shift operations are somewhat misleading, hence they are listed as extended shift instructions on this page.

They are available for use with 16- and 32-bit data entities (registers/memory locations). The src operand is always a register, the dest operand can be a register or memory location, the cnt operand is an immediate byte value or the CL register. In 64-bit mode it is possible to address 64-bit data as well.

shld cnt, src, dest GAS Syntax
shld dest, src, cnt Intel Syntax

The operation performed by shld is to shift the most significant cnt bits out of dest , but instead of filling up the least significant bits with zeros, they are filled with the most significant cnt bits of src .

shrd cnt, src, dest GAS Syntax
shrd dest, src, cnt Intel Syntax

Likewise, the shrd operation shifts the least significant cnt bits out of dest , and fills up the most significant cnt bits with the least significant bits of the src operand.

Intel's nomenclature is misleading, in that the shift does not operate on double the basic operand size (i.e. specifying 32-bit operands doesn't make it a 64-bit shift): the src operand always remains unchanged.

Also, Intel's manual [1] states that the results are undefined when cnt is greater than the operand size, but at least for 32- and 64-bit data sizes it has been observed that shift operations are performed by ( cnt mod n ), with n being the data size.

Examples (GAS Syntax):

xorw %ax,%ax # ax=0000.0000.0000.0000 (0x0000) notw %ax # ax=1111.1111.1111.1111 (0xffff) movw $0x5500,%bx # bx=0101.0101.0000.0000 shrdw $4,%ax,%bx # bx=1111.0101.0101.0000 (0xf550), ax is still 0xffff shldw $8,%bx,%ax # ax=1111.1111.1111.0101 (0xfff5), bx is still 0xf550

Other examples (decimal numbers are used instead of binary number to explain the concept)

# ax = 1234 5678 # bx = 8765 4321 shrd $3, %ax, %bx # ax = 1234 5678 bx = 6788 7654
# ax = 1234 5678 # bx = 8765 4321 shld $3, %ax, %bx # bx = 5432 1123 ax = 1234 5678

Rotate Instructions

Rotate Right

In a rotate instruction, the bits that slide off the end of the register are fed back into the spaces.

ror offset, variable GAS Syntax
ror variable, offset Intel Syntax

Rotate variable to the right by offset bits. Here is a graphical representation how this looks like:

╭─────────╮ %al old │ 0 1 1 1 │ │ │ │ │ ╰─╯ │ │ │ ╰─╮ ror 1, %al │ │ ╰─╮ │ │ ╰─╮ │ │ ╰─╮ │ │ │ %al new 1 0 1 1

The number of bits to rotate offset is masked to the lower 5 bits (or 6 bits in 64-bit mode). This is equivalent to a operation, i. e. the remainder of integer division (note: ).

Operands