internals: Add short description of eumel0 machine

1 files changed, 29 insertions, 0 deletions

diff --git a/internals.rst b/internals.rst
index 5653c2a..5f1b062 100644
--- a/internals.rst
+++ b/internals.rst
@@ -32,3 +32,32 @@ Bootstrapping
 5. Generate initial task tree.
 6. Save all dataspaces to disk.
 
+Virtual machine
+^^^^^^^^^^^^^^^
+
+The EUMEL0 machine is a process virtual machine just like Pascal’s p-code
+machine or the Java Virtual Machine (JVM). It isolates tasks from each other
+and the system. Running native code is not possible.  EUMEL0 is a multi address
+machine with no general purpose registers and thus data lives in virtual memory
+only. A few internal registers like instruction counter, condition flag for
+branching, several segment and status registers and a stack pointer exist.
+
+The CISC instruction set was specifically designed for the high-level language
+ELAN. It includes arithmetic operations for ELAN’s primitive datatypes such as
+signed and unsigned integer as well as float (REAL) and operations for
+bytestrings (TEXT) and dataspaces. However the machine itself does not enforce
+datatypes at runtime. The usual control flow operations are accompanied by
+ELAN-specific call and return instructions. Special instructions for terminal
+and archive disk I/O, exception-like error handling and inter-process
+communication (IPC) are available.
+
+The 54 primary instructions can be encoded with two bytes. They consist of a
+6 bit opcode and one short operand. Whenever the latter is longer than 10 bit a
+long encoding using two bytes plus operands is used. This format also encodes
+42 secondary/special instructions.
+
+Read more instruction encoding details in chapter two of [kernel83]_ and the
+ELAN package `eumel coder`_.
+
+.. _eumel coder: src/basic/eumel%20coder%201.8.1.html
+