## CS 461/561 Computer Architecture

Introduction

A complex number consists of a real and imaginary component and is usually written in the form  where  and  are either integer or floating-point values and  (the imaginary value) . Sometimes in engineering, the letter  is used in place of  because  is used for other values.

Multiplying two complex numbers is done by applying the FOIL (Firsts, Outers, Inners and Lasts) method, similar to that of binomial multiplication.  For example, multiplying (a + bi)(c + di) is accomplished as follows:

Firsts:        a * c

Outers:      a * di

Inners:      bi * c

Lasts:        bi * di

This produces (a+bi)(c+di) = ac + adi + bci + bdi2. The terms are combined to produce the product back in the form a + bi. Keep in mind that i2 = -1.

An example using actual values:  (2.5 + 3i)(4.0 + 2i)

Firsts:        2.5 * 4.0

Outers:      2.5 * 2i

Inners:      3i * 4.0

Lasts:        3i * 2i

This produces 10 + 5i + 12i + 6i2 = 10 + 17i + 6(-1) = 4 + 17i.

Some contemporary programming languages natively support complex numbers (Python, MATLAB).  Newer revisions of some older languages (C, FORTRAN) have added support for complex numbers.  Some programming languages have no native support for complex numbers.

Assignment Definition

Consider the following high-level language code which multiplies two vectors that contain single-precision complex numbers:

Values a, b and c are vectors; _re is the real component element and _im is the imaginary component element in each vector.

1. Convert this loop into pseudo RV64V assembly code using strip mining assuming the following architectural features:

Register s0 = loop counter & array index [i]

Vector registers:  v0 – v31

MVL (maximum vector length) = 64

vst (vector store)

vsub (vector subtract)

vmul (vector multiply)

bne (branch if not equal)*

blt (branch if less than)*

j (unconditional jump)*

ori (logical or immediate)*

Note:  instructions with an asterisk indicate the instructions are used only for setting initial index value and increments, and for loop control.

1. If the vector processor implements chaining with two lanes and has a single vector load/store unit, using the pseudo assembly code from question 1, show how convoys would be constructed to execute in the vector pipeline. How many chimes are required to execute the convoys?

1. Assume in the vector processor, the functional units have the following startup overhead: load/store unit: 12 cycles, multiply unit: 7 cycles, and the add/subtract unit: 6 cycles.  How many clock cycles are required for each iteration of the loop, including startup overhead?

1. How many iterations are required to complete processing the vectors?

Instruction Formats

vld (vector load):                        vld   vD, vec_ref

vst (vector store):                       vst   vD, vec_ref

vsub (vector subtract):                 vsub vD, vS1, vS2

vmul (vector multiply):                 vmul vD, vS1, vS2

bne (branch if not equal):             bne  x1, x2, target_label

blt (branch if less than):               blt  x1, x2, target_label

j (unconditional jump):                 j  target_label

ori (logical or immediate):            ori  xD, xS1, const

Format Definitions

vD = destination vector register

vS1 = first source vector register

vS2 = second source vector register

vec_ref = vector reference (name)

x1 = first general purpose register for comparison

x2 = second general purpose register for comparison

xS1 = first source general purpose register

xS2 = second source general purpose register

target_label = label of the target instruction for branch

const = an integer constant