diff --git a/vvp/vthread.txt b/vvp/vthread.txt
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+
+THREAD DETAILS
+
+Thread objects in vvp are created by ``.thread'' statements in the
+input source file. These cause
+
+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 are for
+use by the arithmetic operators as they operate.
+
+Instructions that use the bit registers have as an operand a <bit>
+value. There is space in the instruction for 2 <bit> operands. The
+special values 0, 1, 2 and 3 are special constant bits 0, 1, x and z
+and are used as immediate values for instructions that take single-bit
+values. The remaining of 64K possible <bit> values are read-write bit
+registers that can be accessed singly or as vectors.