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Maxim Integrated 19-16

MAX31782 User’s Guide

Revision 0; 8/11

19.9 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 pre- fix 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 mne-

monic 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 used by the MAX31782 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, however only the lowest four bits are used and setting SP to 0Fh will return

it to its reset state.

Since the stack is 16 bits wide, it is possible to store two 8-bit register values on it in a single location. This allows more

efficient use of the stack if it is being used to save and restore registers at the start and end of a subroutine.

SubOne:

move PFX[0],IC

push PSF   ;storeIC:PSFonthestack

...

pop GR   ;16-bitregister

move IC,GRH  ;ICwasstoredashighbyte

move PSF,GRL  ;PSFwasstoredaslowbyteret

19.10 Accessing Data Memory

Data memory is accessed through the data pointer registers DP[0] and DP[1] or the Frame Pointer BP[OFFS]. Once one

of these registers is set to a location in data memory, that location can be read or written as follows, using the mnemonic

@DP[0], @DP[1] or @BP[OFFS] as a source or destination.

move DP[0],#0000h  ;setpointertolocation0000h

move A[0],@DP[0]  ;readfromdatamemory

move @DP[0],#55h  ;writetodatamemory

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MAX31782 User’s Guide 1

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