Supervisor call

Your kernel needs a way to expose her management operations to the applications that are running on the userspace.

When an application wants to read a file it has to call the kernel which then will give them something like a file handler or an GUID, or whatever you have in mind, it's your SVC ABI after all.

Now you need to setup a SVC handler:

All of the supervisor calls set the PSW dictated by real locations 0x1C0 (z/Arch) or 0x60 (ESA S/390), with a size of 16 and 8 respectively.

As you have seen on the misc page you can see the structure of both PSW formats, you just have to point to the asm stub of your supervisor call and you are ready to go.

The stub

We are going to first declare our stub and make it globally visible so we can access it from our kernel

.globl svc_handler_stub
svc_handler_stub:

Finally we are going to load our own interrupt stack and jump into the function (the interrupt stack will be initially cleared by an initial entry code, that would clear the bss)

    stmg %r0, %r15, reg_area # Save all registers
    larl %r15, int_stack_top
    brasl %r14, svc_handler # Call our handler
    lmg %r0, %r15, reg_area # Reload all registers
    lpsw 320 # Load old PSW

And this would be our data we will require (not multithread safe, but it's a good start!)

.section .bss
svc_reg_args:
    .skip 128

int_stack_bottom:
    .skip 4096
int_stack_top: