59. FMUL – Fractional Multiply Unsigned
59.1. Description
This instruction performs 8-bit × 8-bit → 16-bit unsigned multiplication and shifts the result one bit left.
Rd Rr R1 R0
Multiplicand
×
Multiplier
→
Product High Product Low
8 8 16
Let (N.Q) denote a fractional number with N binary digits left of the radix point, and Q binary digits right of
the radix point. A multiplication between two numbers in the formats (N1.Q1) and (N2.Q2) results in the
format ((N1+N2).(Q1+Q2)). For signal processing applications, the format (1.7) is widely used for the
inputs, resulting in a (2.14) format for the product. A left shift is required for the high byte of the product to
be in the same format as the inputs. The FMUL instruction incorporates the shift operation in the same
number of cycles as MUL.
The (1.7) format is most commonly used with signed numbers, while FMUL performs an unsigned
multiplication. This instruction is therefore most useful for calculating one of the partial products when
performing a signed multiplication with 16-bit inputs in the (1.15) format, yielding a result in the (1.31)
format. Note: the result of the FMUL operation may suffer from a 2’s complement overflow if interpreted
as a number in the (1.15) format. The MSB of the multiplication before shifting must be taken into
account, and is found in the carry bit. See the following example.
The multiplicand Rd and the multiplier Rr are two registers containing unsigned fractional numbers where
the implicit radix point lies between bit 6 and bit 7. The 16-bit unsigned fractional product with the implicit
radix point between bit 14 and bit 15 is placed in R1 (high byte) and R0 (low byte).
This instruction is not available in all devices. Refer to the device specific instruction set summary.
Operation:
(i) R1:R0 ← Rd × Rr (unsigned (1.15) ← unsigned (1.7) × unsigned (1.7))
Syntax: Operands: Program Counter:
(i) FMUL Rd,Rr 16 ≤ d ≤ 23, 16 ≤ r ≤ 23 PC ← PC + 1
(i) PC ← PC + 1
16-bit Opcode:
0000 0011 0ddd 1rrr
59.2. Status Register (SREG) and Boolean Formula
I T H S V N Z C
– – – – – – ⇔ ⇔
C R16
Atmel AVR Instruction Set Manual [OTHER]
Atmel-0856L-AVR-Instruction-Set-Manual_Other-11/2016
92
To Next Page
Loading...
Need help?
General
