Double Mapped Port Bits

pjvpjv Posts: 1,903
edited 2006-04-13 - 20:54:56 in Propeller 1
Hello Chip;

In certain instances, such as the instruction to select a certain port bit for a counter, the port bit appears to be doubly mapped; once in it's native number, and once·into the space 20 Hex locations above it.

For example to select·portA bit1·for counterA:······ movs··· ctra,#$01
appears also is equally effected by writing: ·········· movs··· ctra,#$21

The ctrA/B pin assignments are selected with 6 bits, permitting port bits 63 ($3F)·through 0 ($00) to be addressed; $3F through $20 belonging to port B, and likewise $1F through $00 to port A.

My question is.. what·other registers/peripherals·are·similarily doubly mapped? I ask this because I'm considering to·use some in an unorthodox manner, and hence we may need to stay alert about them.


Peter (pjv)


  • cgraceycgracey Posts: 13,176
    edited 2006-04-13 - 20:54:56

    I believe this is the only case of 'ghosting' in regards to a future 64-pin version of the current architecture. Oh, wait, the video peripheral has only two bits to specify which 8-bit pin group the video hardware should affect. In the 64-pin version, there will be one more bit added for this purpose (bit 11 in VCFG).


    Chip Gracey
    Parallax, Inc.
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