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<section id="thread-details">
<h1>Thread Details<a class="headerlink" href="#thread-details" title="Link to this heading"></a></h1>
<p>Thread objects in vvp are created by <cite>.thread</cite> statements in the
input source file.</p>
<p>A thread object includes a program counter and private bit
registers. The program counter is used to step the processor through
the code space as it executes instructions. The bit registers each
hold Verilog-style 4-value bits and are for use by the arithmetic
operators as they operate.</p>
<p>The program counter normally increments by one instruction after the
instruction is fetched. If the instruction is a branching instruction,
then the execution of the instruction sets a new value for the pc.</p>
<p>Instructions that use the bit registers have as an operand a &lt;bit&gt;
value. There is usually space in the instruction for 2 &lt;bit&gt;
operands. Instructions that work on vectors pull the vector values
from the bit registers starting with the LSB and up.</p>
<p>The bit addresses 0, 1, 2 and 3 are special constant bits 0, 1, x and
z, and are used as read-only immediate values. If the instruction
takes a single bit operand, then the appropriate value is simply read
out. If the instruction expects a vector, then a vector of the
expected width is created by replicating the constant value.</p>
<p>Bits 4, 5, 6 and 7 are read/write bits but are reserved by many
instructions for special purposes. Comparison operators, for example,
use these as comparison flag bits.</p>
<p>The remaining 64K-8 possible &lt;bit&gt; values are read-write bit registers
that can be accessed singly or as vectors. This obviously implies that
a bit address is 16 bits.</p>
<p>Threads also contain 16 numeric registers. These registers can hold a
real value or a 64bit integer, and can be used in certain cases where
numeric values are needed. The thread instruction set includes
%ix/* instructions to manipulate these registers. The instructions
that use these registers document which register is used, and what the
numeric value is used for. Registers 0-3 are often given fixed
meanings to instructions that need an integer value.</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span><span data-line="1">/*
</span><span data-line="2"> * Copyright (c) 2001-2024 Stephen Williams (steve@icarus.com)
</span><span data-line="3"> *
</span><span data-line="4"> * This source code is free software; you can redistribute it
</span><span data-line="5"> * and/or modify it in source code form under the terms of the GNU
</span><span data-line="6"> * General Public License as published by the Free Software
</span><span data-line="7"> * Foundation; either version 2 of the License, or (at your option)
</span><span data-line="8"> * any later version.
</span><span data-line="9"> *
</span><span data-line="10"> * This program is distributed in the hope that it will be useful,
</span><span data-line="11"> * but WITHOUT ANY WARRANTY; without even the implied warranty of
</span><span data-line="12"> * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
</span><span data-line="13"> * GNU General Public License for more details.
</span><span data-line="14"> *
</span><span data-line="15"> * You should have received a copy of the GNU General Public License
</span><span data-line="16"> * along with this program; if not, write to the Free Software
</span><span data-line="17"> * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
</span><span data-line="18"> */
</span></pre></div>
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