Maxim Integrated DS4830A Optical Microcontroller - Manual (Page 200)

DS4830A User’s Guide
200
...
(interrupt servicing code)
...
pop IMR
; restore previous interrupt mask
ret ; back to code or lower priority interrupt
Note that configuring a given IMR register mask bit to '0' only prevents interrupt conditions from the corresponding
module or system from generating an interrupt request. Configuring an IMR mask bit to '0' does not prevent the
corresponding IIR system or module identification flag from being set. This means that when using the IMR mask
register functionality to block interrupts, there may be cases when both the mask (IMR.x) and identifier (IIR.x) bits
should be considered when determining if the corresponding peripheral should be serviced.
23.8.1 – Conditional Return from Interrupt
Similar to the conditional returns, the DS4830A microcontroller also supports a set of conditional return from interrupt
operation. Based upon the value of one of the status flags, the CPU can conditionally pop the stack, clear the INS bit
to 0, and begin execution at the address popped from the stack. If the condition is not true, the conditional return
from interrupt instruction leaves the INS bit unchanged, does not pop the stack and does not change the instruction
pointer. The following conditional return from interrupt operations are supported:
RETI C ; if C=1, a RETI is executed
RETI NC ; if C=0, a RETI is executed
RETI Z ; if Z=1 (Acc=00h), a RETI is executed
RETI NZ ; if Z=0 (Acc<>00h), a RETI is executed
RETI S ; if S=1, a RETI is executed
23.9 – Accessing the Stack
The hardware stack is used automatically by the CALL, RET and RETI instructions, but it can also be used explicitly
to store and retrieve data. All values stored on the stack are 16 bits wide.
The PUSH instruction increments the stack pointer SP and then stores a value on the stack. When pushing a 16-bit
value onto the stack, the entire value is stored. However, when pushing an 8-bit value onto the stack, the high byte
stored on the stack comes from the prefix register. The @++SP stack access mnemonic is the associated
destination specifier that generates this push behavior, thus the following two instruction sequences are equivalent:
move PFX[0], IC
push PSF
; stored on stack: IC:PSF
move PFX[0], IC
move @++SP, PSF
; stored on stack: IC:PSF
The POP instruction removes a value from the stack and then decrements the stack pointer. The @SP-- stack
access mnemonic is the associated source specifier that generates this behavior, thus the following two instructions
are equivalent:
pop PSF
move PSF, @SP--
The POPI instruction is equivalent to the POP instruction but additionally clears the INS bit to ‘0’. Thus, the following
two instructions would be equivalent:
popi IP
reti
The @SP-- mnemonic can be utilized by the DS4830A microcontroller so that stack values may be used directly by
ALU operations (e.g. ADD src, XOR src, etc.) without requiring that the value be first popped into an intermediate
register or accumulator.
add @SP-- ; sum the last three words pushed onto the stack
add @SP-- ; with Acc, disregarding overflow
add @SP--
The stack pointer SP can be set explicitly. For a DS4830A, which has a stack depth of 16 words, only the lowest
four bits are used and setting SP to 0Fh will return it to its reset state.