sw-gloss-arm/angel (libgloss.la :: gloss_component_library)


This component emulates the Angel ROM monitor software.



This component emulates the ANGEL ROM monitor software by intercepting the ARM CPU component model's software interrupt (SWI). It implements common system calls, such as read() and write().

system call emulation

When the trap input pin is driven, this component will attempt to emulate the requested system call. It checks whether the current trap-code and trap values correspond to a system call that can be emulated.

If this is indeed a valid ANGEL system call, the component emulates the call by accessing the target processor's registers (through the cpu relation) and memory (through the target-memory accessor). In most cases, it signals back a skip or reissue indication through the trap output pin.

If the requested system call is not a valid ANGEL system call, the trap signal is chained by driving the trap-code-chain and trap-chain output pins with the same values as received. If an external component is connected to these chain pins, and if that external component signals a handled indication back on the trap-chain input, then this component will consider the trap as handled and will return.

If this is a trap condition that would kill an ordinary process-model program, then the process-signal output pin is driven with an exit code similar to that used by UNIX wait(2). Such conditions include memory faults, invalid instructions, etc.

See the documentation of hw-cpu-arm7t for the list of supported trap & trap-code values.

If the verbose? attribute is set to a true value, detailed tracing messages will be printed to stderr during a system call emulation.

The syscall-numbering-scheme setting causes the gloss component to use the system call numbering scheme specified. The gloss component may then properly interpret non-standard syscall numbers that come from target-program libgloss routines. Currently, the only available non-standard numbering scheme is cygmon.


When the reset pin is driven, all open files are closed and the simulated stdio file handles (0/1/2) are attached to the host's same stdio file handles.

The heap-base, heap-limit, stack-base, stack-limit, and command-line attributes provide access to values that are exposed to several ANGEL system calls.

The max-fds attribute specifies the largest simulated file descriptor that will be made available to the target program.


Accesses to the special simulated file handles 0/1/2 are mapped to pin I/O on the debug-tx and debug-rx pins. Input from the debug-rx pin is buffered for future reads. Reads may block until data is received. Writes to stdout or stderr are modelled by sending the supplied characters, one-at-a-time, out through the debug-tx pin.

SID Conventions
supervisory supported -
triggerpointsnot supported -
save/restore supported -
inhibit-recursionnot supported -


Related components

The Angel component communicates closely with a CPU and main memory. It uses the CPU for retrieving system call arguments from registers and placing results in a register. It uses memory for accessing data when system call arguments are pointers into the target memory. An example of this is the char* argument to write(). The coupling to the CPU and memory is quite relaxed: the only requirement is that the endianness of the CPU be known via attribute and that attributes for registers exist with names of the form rN.

The configuration file fragment below shows how to connect an Angel component into a more complete system:

	new hw-cpu-arm7t cpu
	new hw-memory-ram/rom-basic mem
	new sw-gloss-arm/angel angel
	new sid-io-stdio stdio
	relate angel cpu cpu
	connect-pin cpu trap <-> angel trap
	connect-pin cpu trap-code -> angel trap-code
	connect-pin angel process-signal -> main stop!
	connect-pin angel debug-tx -> stdio stdout
	connect-pin angel debug-rx <- stdio stdin
	connect-bus angel target-memory cpu debugger-port

Host system

The component implements the Angel's semi-hosted system calls such as open() and close(). Use these calls with caution: they can perform file I/O on the host, therefore it is possible to write rogue target programs that delete or modify files on the host.

Component Reference:

Component: sw-gloss-arm/angel

traptrap type enumsystem call emulation
trap-codeintrap code valuesystem call emulation
process-signaloutsignal numbersystem call emulation

namecategorylegal valuesdefault valuebehaviors
verbose?settingbooleanfalsesystem call emulation
syscall-numbering-schemesettingstringlibglosssystem call emulation
'tk tty'gui-'hw-visual-tty'component gui

target-memoryanysystem call emulation


ARM Software Development Toolkit Reference Guide (version 2.11), Document number: ARM DUI 0041B, Chapter 8 (``Angel'')

newlib source code is also useful for clarification.