CHM Instruction

CHM Instructions

Purpose:

To request services of more priviledged software

Format:

opcode code.rw

Operation:

If PSL<IS> EQLU 1 then: HALT; Illegal from Interrupt Stack.
if opcode-mode LSSU PSL<CMD> then: PSL<CMD>_SP <- SP;; SP <- opcode-mode_SP;
-(SP) <- PSL;
-(SP) <- PC;
-(SP) <- sign-extended code;
PSL<CM,TP,FPD,DV,FU,IV,T,N,Z,V,C> <- 0;
PSL<PMD> <- PSL<CMD>
if opcode-mode LSSU PSL<CMD> then: PSL<CMD>_SP <- opcode-mode;
PC <- {SCB vector for opcode-mode};

Condition
Codes:

N <- 0;
Z <- 0;
V <- 0;
C <- 0;

Exceptions:

halt

Opcodes:

BC	CHMK	Change mode to Kernel
BD	CHME	Change mode to Executive
BE	CHMS	Change mode to Supervisor
BF	CHMU	Change mode to User

Description:

Change mode instructions all processes to change their access mode in a controlled manner. The instruction only increases priviledge mode (i.e. decreases the access mode)

A change in mode also results in a change of stack pointers; the old stack pointer is saved, the new pointer is loaded. The PSL, PC and code operand are pushed onto the stack of the new mode. The saved PC addresses the instruction following the CHMx instruction. The code is sign-extended to a longword. After Execution the stack looks like:

Sign Extended Code	:SP
PC of Instruction After CHMx
Old PSL

The destination mode selected by the opcode is used to select a location from the System Control Block. This location addresses the CHMx dispatcher for the specified mode.