Syscall Table

Syscalls are the interface between user-space programs and the JSOS kernel. They are invoked from Yog code with the syscall() intrinsic, which yogc compiles to a SVC #0 instruction.

Phase 2+ — not yet implemented. The table below documents the planned interface. Phase 1 has no kernel/user boundary; all code runs in EL1.

How syscalls work

// User code
const n = syscall(0, 1, buf, len);   // write(fd=1, buf, len) → bytes written

yogc emits:

; Arguments already in x0–x2 (placed by caller)
movz x8, #0          ; syscall number
svc  #0              ; trap to EL1 kernel SVC handler
; return value in x0

The kernel SVC handler (Phase 2) reads x8 for the syscall number and dispatches to the appropriate handler. Return values are placed in x0 before eret.

Phase 2 — Core I/O

Number	Name	Signature	Description
0	write	(fd, buf, len) → n	Write len bytes from buf to file descriptor fd
1	read	(fd, buf, len) → n	Read up to len bytes from fd into buf
2	open	(path) → fd	Open a file; returns file descriptor or negative error
3	fwrite	(fd, buf, len) → n	Buffered write
4	fread	(fd, buf, len) → n	Buffered read
5	spawn	(bin, len) → pid	Load binary image of len bytes from bin and start a new process
6	exit	(code)	Terminate the calling process with exit code code
7	wait	(pid) → code	Block until process pid exits; return its exit code

Well-known file descriptors:

fd	Target
0	stdin (UART RX)
1	stdout (UART TX)
2	stderr (UART TX)

Phase 5 — Memory management

Number	Name	Signature	Description
8	mmap	(addr, len, prot) → ptr	Map len bytes at hint address addr; returns actual address
9	munmap	(addr, len) → 0	Unmap a previously mapped region
10	brk	(addr) → addr	Set program break (end of heap segment)

prot flags (OR together):

Value	Meaning
0x1	PROT_READ
0x2	PROT_WRITE
0x4	PROT_EXEC

Phase 7 — IPC and signals

Number	Name	Signature	Description
11	pipe	(fds)	Create a pipe; write fd and read fd placed in fds[0] and fds[1]
12	signal	(sig, handler)	Register handler as the signal handler for signal number sig
13	kill	(pid, sig) → 0	Send signal sig to process pid
14	sigreturn	()	Return from a signal handler; restores saved register state

Calling convention

JSOS follows the AArch64 procedure call standard (AAPCS64) for syscalls:

• Arguments in x0–x5 (up to 6)
• Syscall number in x8
• Return value in x0
• Negative return values indicate errors (errno-style)
• Registers x0–x17 are caller-saved across SVC

The syscall() intrinsic in Yog handles argument placement and number loading. Direct SVC instructions are not part of the Yog dialect.