Maxim-integrated MAXQ622 Manual de usuario descargar pdf (Pagina 246)

Maxim Integrated A1-1

MAXQ612/MAXQ622 User’s Guide

APPENDIX 1: DATA POINTER USAGE EXAMPLES

IMPORTANT: MAXQ20 family pointer mode (DPC.WBS) bits and source select

(DPC.SDPS) bits should not be changed simultaneously.

Writing immediate values to DPC (e.g. MOVE DPC, #4)

without knowing the previous contents can implicitly

setup a situation where the mode and select bits are

changed simultaneously. A function call with

foo:

push DPC

;...

move DPC, #4

;...

pop DPC

ret

will, in many cases, setup a situation where mode and

select bits are changed simultaneously. If you are

ever in doubt about the contents of DPC then read the

contents, change only the desired bits using a bit mask and

write the modified value back to DPC.

NOTE: Compilers already manage DPC correctly. These examples are intended to benefit

people that write assembly language routines, modules or applications.

NOTE: All data pointers are implemented with 17 bits, with sliding windows that expose

the upper 16 bits when the corresponding DPC.WBS is set to 1 (word mode) and expose

the lower 16 bits if the corresponding DPC.WBS is cleared to 0 (byte mode).

MAXQ 17-bit Internal Pointer Representation (DP[0], DP[1], BP[OFFS]

[, CP where applicable]):

internal bit position: 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

word mode window: ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++

byte mode window: ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++

If you need to preserve a pointer across an interrupt service routine

or a function call and you modify the pointer’s mode then you must

preserve all 17 bits of any pointer used. The most basic form of this

operation looks like (DP[0] example)

foo:

push DPC ; preserve DPC

move DPC, #0h ; save the

push DP[0] ; byte mode version of DP[0]

move DPC, #4h ; save the

push DP[0] ; word mode version of DP[0]

;

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Maxim-integrated MAXQ622 Manual de usuario Pagina 246