GPUSH

Push a garbage collector root onto the GP stack.

Opcode	Bytes	Cycles	Form	Example
$1e	1	2	gpush pushed	gpush pushed
$34	2	3	gpush i:_	gpush i:4
$35	2	3	gpush null i:_	gpush null i:4

Notes

• The GP stack is used by the Hybrix memory manager to track GC roots for the purpose of garbage collection (GC). Each GC root stores the memory address of a pointer variable that should be analyzed by the garbage collector.

• The GP stack grows backwards (pushing decreases the gp register), whereas the SP stack grows forwards. (The "top" of the GP stack is actually the lowest memory address.)

• Since the Hybrix hardware uses a 24-bit memory bus, Chombit's GP stack is configured to store 3-byte trio pointers. When pushing a larger value, the extra bits are discarded.

• The effect of gpush i:4 for example is to push i:4 onto the GP stack. It first stores gp - 3 in gp. Then it stores the address of i:4 (not the value of i:4) at the memory address pointed to by gp.

• gpush null i:4 for example performs all those actions and then also assigns 0 to i:4. This form is used to initialize a local variable to contain a null pointer.

• gpush pushed works differently: It first stores gp - 3 in gp, then assigns sp - 4 to the memory address pointed to by gp. sp - 4 is assumed to be a recently pushed int pointer. For example, if we just did push i:4, then gpush pushed is equivalent to gpush i:4.

• If io::stack_guard is 1, then gpush will trigger a stack fault event if the sp register becomes larger than the gp register.

• gpop undoes the action of gpush.

• This instruction does not affect the CPU condition flags.