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+TCG Interpreter (TCI) - Copyright (c) 2011 Stefan Weil.
+
+This file is released under the BSD license.
+
+1) Introduction
+
+TCG (Tiny Code Generator) is a code generator which translates
+code fragments ("basic blocks") from target code (any of the
+targets supported by QEMU) to a code representation which
+can be run on a host.
+
+QEMU can create native code for some hosts (arm, hppa, i386, ia64, ppc, ppc64,
+s390, sparc, x86_64). For others, unofficial host support was written.
+
+By adding a code generator for a virtual machine and using an
+interpreter for the generated bytecode, it is possible to
+support (almost) any host.
+
+This is what TCI (Tiny Code Interpreter) does.
+
+2) Implementation
+
+Like each TCG host frontend, TCI implements the code generator in
+tcg-target.c, tcg-target.h. Both files are in directory tcg/tci.
+
+The additional file tcg/tci.c adds the interpreter.
+
+The bytecode consists of opcodes (same numeric values as those used by
+TCG), command length and arguments of variable size and number.
+
+3) Usage
+
+For hosts without native TCG, the interpreter TCI must be enabled by
+
+        configure --enable-tcg-interpreter
+
+If configure is called without --enable-tcg-interpreter, it will
+suggest using this option. Setting it automatically would need
+additional code in configure which must be fixed when new native TCG
+implementations are added.
+
+System emulation should work on any 32 or 64 bit host.
+User mode emulation might work. Maybe a new linker script (*.ld)
+is needed. Byte order might be wrong (on big endian hosts)
+and need fixes in configure.
+
+For hosts with native TCG, the interpreter TCI can be enabled by
+
+        configure --enable-tcg-interpreter
+
+The only difference from running QEMU with TCI to running without TCI
+should be speed. Especially during development of TCI, it was very
+useful to compare runs with and without TCI. Create /tmp/qemu.log by
+
+        qemu-system-i386 -d in_asm,op_opt,cpu -D /tmp/qemu.log -singlestep
+
+once with interpreter and once without interpreter and compare the resulting
+qemu.log files. This is also useful to see the effects of additional
+registers or additional opcodes (it is easy to modify the virtual machine).
+It can also be used to verify native TCGs.
+
+Hosts with native TCG can also enable TCI by claiming to be unsupported:
+
+        configure --cpu=unknown --enable-tcg-interpreter
+
+configure then no longer uses the native linker script (*.ld) for
+user mode emulation.
+
+
+4) Status
+
+TCI needs special implementation for 32 and 64 bit host, 32 and 64 bit target,
+host and target with same or different endianness.
+
+            | host (le)                     host (be)
+            | 32             64             32             64
+------------+------------------------------------------------------------
+target (le) | s0, u0         s1, u1         s?, u?         s?, u?
+32 bit      |
+            |
+target (le) | sc, uc         s1, u1         s?, u?         s?, u?
+64 bit      |
+            |
+target (be) | sc, u0         sc, uc         s?, u?         s?, u?
+32 bit      |
+            |
+target (be) | sc, uc         sc, uc         s?, u?         s?, u?
+64 bit      |
+            |
+
+System emulation
+s? = untested
+sc = compiles
+s0 = bios works
+s1 = grub works
+s2 = Linux boots
+
+Linux user mode emulation
+u? = untested
+uc = compiles
+u0 = static hello works
+u1 = linux-user-test works
+
+5) Todo list
+
+* TCI is not widely tested. It was written and tested on a x86_64 host
+  running i386 and x86_64 system emulation and Linux user mode.
+  A cross compiled QEMU for i386 host also works with the same basic tests.
+  A cross compiled QEMU for mipsel host works, too. It is terribly slow
+  because I run it in a mips malta emulation, so it is an interpreted
+  emulation in an emulation.
+  A cross compiled QEMU for arm host works (tested with pc bios).
+  A cross compiled QEMU for ppc host works at least partially:
+  i386-linux-user/qemu-i386 can run a simple hello-world program
+  (tested in a ppc emulation).
+
+* Some TCG opcodes are either missing in the code generator and/or
+  in the interpreter. These opcodes raise a runtime exception, so it is
+  possible to see where code must be added.
+
+* The pseudo code is not optimized and still ugly. For hosts with special
+  alignment requirements, it needs some fixes (maybe aligned bytecode
+  would also improve speed for hosts which support byte alignment).
+
+* A better disassembler for the pseudo code would be nice (a very primitive
+  disassembler is included in tcg-target.c).
+
+* It might be useful to have a runtime option which selects the native TCG
+  or TCI, so QEMU would have to include two TCGs. Today, selecting TCI
+  is a configure option, so you need two compilations of QEMU.