15
15 – 85
MISC
STACK CONTROL
TOPPCSTACK
A special version of the Register-to-Register Move instruction, Type 17, is provided
for reading (and popping) or writing (and pushing) the top value of the PC stack.
The normal POP PC instruction does not save the value popped from the stack, so to
save this value into a register you must use the following special instruction:
reg = TOPPCSTACK; {pop PC stack into reg}
{“toppcstack” may also be lowercase}
The PC stack is also popped by this instruction, after a one-cycle delay. A NOP
should usually be placed after the special instruction, to allow the pop to occur
properly:
reg = TOPPCSTACK;
NOP; {allow pop to occur correctly}
There is no standard PUSH PC stack instruction. To push a specific value onto the
PC stack, therefore, use the following special instruction:
TOPPCSTACK = reg; {push reg contents onto PC stack}
The stack is pushed immediately, in the same cycle.
Note that “TOPPCSTACK” may not be used as a register in any other instruction type!
Examples:
AX0 = TOPPCSTACK; {pop PC stack into AX0}
NOP;
TOPPCSTACK = I7; {push contents of I7 onto PC stack}
Only the following registers may be used in the special TOPPCSTACK instructions:
ALU, MAC, & Shifter Registers DAG Registers
AX0 AR SI I0 I4 M0 M4 L0 L4
AX1 MR0 SE I1 I5 M1 M5 L1 L5
MX0 MR1 SR0 I2 I6 M2 M6 L2 L6
MX1 MR SR1 I3 I7 M3 M7 L3 L7
AY0
AY1
MY0
MY1
There are several restrictions on the use of the special TOPPCSTACK instructions;
they are described in Chapter 3, Program Control.
(instruction continues on next page)
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