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[[mstandard]]
== "M" Extension for Integer Multiplication and Division, Version 2.0
This chapter describes the standard integer multiplication and division
instruction extension, which is named "M" and contains instructions
that multiply or divide values held in two integer registers.
[TIP]
====
We separate integer multiply and divide out from the base to simplify
low-end implementations, or for applications where integer multiply and
divide operations are either infrequent or better handled in attached
accelerators.
====
=== Multiplication Operations
include::images/wavedrom/m-st-ext-for-int-mult.adoc[]
[[m-st-ext-for-int-mult]]
//.Multiplication operation instructions
(((MUL, MULH)))
(((MUL, MULHU)))
(((MUL, MULHSU)))
MUL performs an XLEN-bit×XLEN-bit multiplication of
_rs1_ by _rs2_ and places the lower XLEN bits in the destination
register. MULH, MULHU, and MULHSU perform the same multiplication but
return the upper XLEN bits of the full 2×XLEN-bit
product, for signed×signed,
unsigned×unsigned, and _rs1_×unsigned _rs2_ multiplication, respectively.
If both the high and low bits of the same product are required, then the recommended code sequence is: MULH[[S]U] _rdh, rs1, rs2_; MUL _rdl, rs1, rs2_ (source register specifiers must be in same order and _rdh_ cannot be the same as _rs1_ or _rs2_). Microarchitectures can then fuse these into a single multiply operation instead of performing two separate multiplies.
[NOTE]
====
MULHSU is used in multi-word signed multiplication to multiply the
most-significant word of the multiplicand (which contains the sign bit)
with the less-significant words of the multiplier (which are unsigned).
====
MULW is an RV64 instruction that multiplies the lower 32 bits of the
source registers, placing the sign extension of the lower 32 bits of the
result into the destination register.
[NOTE]
====
In RV64, MUL can be used to obtain the upper 32 bits of the 64-bit
product, but signed arguments must be proper 32-bit signed values,
whereas unsigned arguments must have their upper 32 bits clear. If the
arguments are not known to be sign- or zero-extended, an alternative is
to shift both arguments left by 32 bits, then use MULH[[S]U].
====
=== Division Operations
include::images/wavedrom/division-op.adoc[]
[[division-op]]
//.Division operation instructions
(((MUL, DIV)))
(((MUL, DIVU)))
DIV and DIVU perform an XLEN bits by XLEN bits signed and unsigned
integer division of _rs1_ by _rs2_, rounding towards zero. REM and REMU
provide the remainder of the corresponding division operation. For REM,
the sign of a nonzero result equals the sign of the dividend.
[NOTE]
====
For both signed and unsigned division, except in the case of overflow, it holds
that
latexmath:[$\textrm{dividend} = \textrm{divisor} \times \textrm{quotient} + \textrm{remainder}$].
====
If both the quotient and remainder are required from the same division,
the recommended code sequence is: DIV[U] _rdq, rs1, rs2_; REM[U] _rdr,
rs1, rs2_ (_rdq_ cannot be the same as _rs1_ or _rs2_).
Microarchitectures can then fuse these into a single divide operation
instead of performing two separate divides.
DIVW and DIVUW are RV64 instructions that divide the lower 32 bits of
_rs1_ by the lower 32 bits of _rs2_, treating them as signed and
unsigned integers respectively, placing the 32-bit quotient in _rd_,
sign-extended to 64 bits. REMW and REMUW are RV64 instructions that
provide the corresponding signed and unsigned remainder operations
respectively. Both REMW and REMUW always sign-extend the 32-bit result
to 64 bits, including on a divide by zero.
(((MUL, div by zero)))
The semantics for division by zero and division overflow are summarized
in <<divby0>>. The quotient of division by zero has all bits
set, and the remainder of division by zero equals the dividend. Signed
division overflow occurs only when the most-negative integer is divided
by latexmath:[$-1$]. The quotient of a signed division with overflow is
equal to the dividend, and the remainder is zero. Unsigned division
overflow cannot occur.
[[divby0]]
.Semantics for division by zero and division overflow. L is the width of the operation in bits: XLEN for DIV[U] and REM[U], or 32 for DIV[U]W and REM[U]W.
[cols="<2,^,^,^,^,^,^",options="header",]
|===
|Condition |Dividend |Divisor |DIVU[W] |REMU[W] |DIV[W] |REM[W]
|Division by zero +
Overflow (signed only) |latexmath:[$x$] +
latexmath:[$-2^{L-1}$] |0 +
latexmath:[$-1$] |latexmath:[$2^{L}-1$] +
- |latexmath:[$x$] +
- |latexmath:[$-1$] +
latexmath:[$-2^{L-1}$] +
|latexmath:[$x$] +
0
|===
//|Overflow (signed only) |latexmath:[$-2^{L-1}$] |latexmath:[$-1$] |– |– |latexmath:[$-2^{L-1}$] |0
//|===
[TIP]
====
We considered raising exceptions on integer divide by zero, with these
exceptions causing a trap in most execution environments. However, this
would be the only arithmetic trap in the standard ISA (floating-point
exceptions set flags and write default values, but do not cause traps)
and would require language implementors to interact with the execution
environment's trap handlers for this case. Further, where language
standards mandate that a divide-by-zero exception must cause an
immediate control flow change, only a single branch instruction needs to
be added to each divide operation, and this branch instruction can be
inserted after the divide and should normally be very predictably not
taken, adding little runtime overhead.
The value of all bits set is returned for both unsigned and signed
divide by zero to simplify the divider circuitry. The value of all 1s is
both the natural value to return for unsigned divide, representing the
largest unsigned number, and also the natural result for simple unsigned
divider implementations. Signed division is often implemented using an
unsigned division circuit and specifying the same overflow result
simplifies the hardware.
====
=== Zmmul Extension, Version 1.0
The Zmmul extension implements the multiplication subset of the M
extension. It adds all of the instructions defined in
<<Multiplication Operations>>, namely: MUL, MULH, MULHU,
MULHSU, and (for RV64 only) MULW. The encodings are identical to those
of the corresponding M-extension instructions. M implies Zmmul.
(((MUL, Zmmul)))
[NOTE]
====
The *Zmmul* extension enables low-cost implementations that require
multiplication operations but not division. For many microcontroller
applications, division operations are too infrequent to justify the cost
of divider hardware. By contrast, multiplication operations are more
frequent, making the cost of multiplier hardware more justifiable.
Simple FPGA soft cores particularly benefit from eliminating division
but retaining multiplication, since many FPGAs provide hardwired
multipliers but require dividers be implemented in soft logic.
====
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