Emulation
QEMU’s Tiny Code Generator (TCG) provides the ability to emulate a number of CPU architectures on any supported host platform. Both System Emulation and User Mode Emulation are supported depending on the guest architecture.
Architecture (qemu name) |
System |
User |
Notes |
|---|---|---|---|
Alpha |
Yes |
Yes |
Legacy 64 bit RISC ISA developed by DEC |
Arm (arm, aarch64) |
Yes |
Wide range of features, see A-profile CPU architecture support for details |
|
AVR |
No |
8 bit micro controller, often used in maker projects |
|
Cris |
Yes |
Yes |
Embedded RISC chip developed by AXIS |
Hexagon |
No |
Yes |
Family of DSPs by Qualcomm |
PA-RISC (hppa) |
Yes |
Yes |
A legacy RISC system used in HP’s old minicomputers |
x86 (i386, x86_64) |
Yes |
The ubiquitous desktop PC CPU architecture, 32 and 64 bit. |
|
Loongarch |
Yes |
Yes |
A MIPS-like 64bit RISC architecture developed in China |
m68k |
Yes |
Motorola 68000 variants and ColdFire |
|
Microblaze |
Yes |
Yes |
RISC based soft-core by Xilinx |
MIPS (mips*) |
Yes |
Venerable RISC architecture originally out of Stanford University |
|
Nios2 |
Yes |
Yes |
32 bit embedded soft-core by Altera |
OpenRISC |
Yes |
Open source RISC architecture developed by the OpenRISC community |
|
Power (ppc, ppc64) |
Yes |
A general purpose RISC architecture now managed by IBM |
|
RISC-V |
Yes |
An open standard RISC ISA maintained by RISC-V International |
|
RX |
No |
A 32 bit micro controller developed by Renesas |
|
s390x |
Yes |
A 64 bit CPU found in IBM’s System Z mainframes |
|
sh4 |
Yes |
Yes |
A 32 bit RISC embedded CPU developed by Hitachi |
SPARC (sparc, sparc64) |
Yes |
A RISC ISA originally developed by Sun Microsystems |
|
Tricore |
Yes |
No |
A 32 bit RISC/uController/DSP developed by Infineon |
Xtensa |
Yes |
A configurable 32 bit soft core now owned by Cadence |
A number of features are only available when running under emulation including Record/Replay and QEMU TCG Plugins.
Semihosting
Semihosting is a feature defined by the owner of the architecture to
allow programs to interact with a debugging host system. On real
hardware this is usually provided by an In-circuit emulator (ICE)
hooked directly to the board. QEMU’s implementation allows for
semihosting calls to be passed to the host system or via the
gdbstub.
Generally semihosting makes it easier to bring up low level code before a more fully functional operating system has been enabled. On QEMU it also allows for embedded micro-controller code which typically doesn’t have a full libc to be run as “bare-metal” code under QEMU’s user-mode emulation. It is also useful for writing test cases and indeed a number of compiler suites as well as QEMU itself use semihosting calls to exit test code while reporting the success state.
Semihosting is only available using TCG emulation. This is because the instructions to trigger a semihosting call are typically reserved causing most hypervisors to trap and fault on them.
Warning
Semihosting inherently bypasses any isolation there may be between
the guest and the host. As a result a program using semihosting can
happily trash your host system. Some semihosting calls (e.g.
SYS_READC) can block execution indefinitely. You should only
ever run trusted code with semihosting enabled.
Redirection
Semihosting calls can be re-directed to a (potentially remote) gdb
during debugging via the gdbstub. Output to the
semihosting console is configured as a chardev so can be
redirected to a file, pipe or socket like any other chardev
device.
Supported Targets
Most targets offer similar semihosting implementations with some minor changes to define the appropriate instruction to encode the semihosting call and which registers hold the parameters. They tend to presents a simple POSIX-like API which allows your program to read and write files, access the console and some other basic interactions.
For full details of the ABI for a particular target, and the set of calls it provides, you should consult the semihosting specification for that architecture.
Note
QEMU makes an implementation decision to implement all file
access in O_BINARY mode. The user-visible effect of this is
regardless of the text/binary mode the program sets QEMU will
always select a binary mode ensuring no line-terminator conversion
is performed on input or output. This is because gdb semihosting
support doesn’t make the distinction between the modes and
magically processing line endings can be confusing.
Architecture |
Modes |
Specification |
|---|---|---|
Arm |
System and User-mode |
https://github.com/ARM-software/abi-aa/blob/main/semihosting/semihosting.rst |
m68k |
System |
https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=libgloss/m68k/m68k-semi.txt;hb=HEAD |
MIPS |
System |
Unified Hosting Interface (MD01069) |
Nios II |
System |
|
RISC-V |
System and User-mode |
https://github.com/riscv/riscv-semihosting-spec/blob/main/riscv-semihosting-spec.adoc |
Xtensa |
System |
Tensilica ISS SIMCALL |