Chapter 9- TRAP Routines & Subroutines

how are subroutines different from TRAP routines

subroutines are not part of the OS

JSR

jumps to a location and saves current PC (address of next instruction) in R7

JSR diagram

JSRR

just like JSR, except it uses different addressing mode

difference between JSR and JSRR

JSRR uses the contents a register as the address to jump to (16 bits), while JSR instruction provides an 11 bit offset to PC

JSRR diagram

RET

this is really a JMP R7

returns to the instruction following the JSR(R)

arguments

the value that is PASSED IN to a subroutine

return values

the value PASSED OUT of a subroutine

JSRR diagram

system call

1. user program invokes system call

2. operating system code performs operation

3. returns control to the user program

this is done through TRAP mechanism

TRAP diagram

TRAP operation

LOOKUP starting address

TRANSFER to service routine

RETURN (JMP R7)