Add ARM semihosting SYS_WRITE (0x05) operation support

[millerjef]

Summary

Implements the missing SYS_WRITE (0x05) semihosting operation to complete ARM semihosting support in Renode.

This enables printf() and other standard I/O functions to work correctly for ARM firmware compiled with newlib's rdimon.specs semihosting support.

Background

ARM semihosting allows embedded firmware to perform I/O operations by making supervisor calls that the debugger/emulator handles. Renode already has partial semihosting support with SemihostingUart, implementing:

• ✓ SYS_READC (0x07) - Read single character
• ✓ SYS_WRITEC (0x03) - Write single character
• ✓ SYS_WRITE0 (0x04) - Write null-terminated string

However, when firmware uses standard newlib functions like printf(), the C library's _write() implementation calls SYS_WRITE (0x05) instead of the simpler operations above. Without this operation, printf-based output doesn't work.

Changes

File: src/Emulator/Cores/Arm/Arm.cs

Added case 5 (SYS_WRITE) handler in the DoSemihosting() method:

• Uses TryTranslateAddress() with proper error handling for robustness
• Reads ARM semihosting parameter block from memory containing:
  • word[0]: File handle (1 = stdout, 2 = stderr)
  • word[1]: Data pointer (memory address of buffer)
  • word[2]: Byte count (number of bytes to write)
• Transfers data from emulated memory to SemihostingUart
• Returns 0 (success) to firmware indicating all bytes were written
• Also improved error handling for existing SYS_WRITEC/SYS_WRITE0 operations

Testing

Tested with STM32 firmware using FreeRTOS and semihosting-based console output. Printf statements now appear correctly in Renode's UART output.

Related Issues

• Addresses #681 - ARM Semihosting support request
• Related to #699 - Rust semihosting UART access

ARM Semihosting Specification

Per ARM Semihosting Documentation, SYS_WRITE (0x05) writes to a file descriptor or stream, with register r1 pointing to a parameter block containing file handle, data pointer, and length.

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