18.2.2 SVCs for InputOutput
When a user-mode program wants to read a typed character it executes a ReadKey() SVC. The binary representation of the SVC has an illegal value in the opcode field, so the CPU hardware causes an exception, which starts executing the illegal opcode handler in the OS. The OS handler recognizes the illegal opcode value as being an SVC and uses the low-order bits of the SVC instruction to determine which sub-handler to call. Here's our first draft for the ReadKey sub-handler, this time written in C. The handler starts by looking at the process table entry for the current process to determine which keyboard buffer holds the characters for the process. Let's assume for the moment the buffer is *not* empty and skip to the last line, which reads the character from the buffer and uses it to replace the saved value for the user's R0 in the array holding the saved register values. When the handler exits, the OS will reload the saved registers and resume execution of the ...