Story 4.3 — SVC syscall path

[Abhinkop]

Goal

Implement a syscall dispatch path so that user programs can request kernel services safely.

Tasks

• Add SVC #0 entry in vector.s: save GPRs, call C dispatcher
• Define initial syscall table: SYS_WRITE=1, SYS_EXIT=60, SYS_GETPID=39
• Implement sys_write(fd, buf_va, len): validate user VA, write to UART
• Implement sys_exit(code): mark task dead, schedule
• Implement sys_getpid(): return current task ID
• Add user-side syscall.h with inline wrappers
• Test: user task prints "Hello from EL0" via sys_write