Seth Fischer <fischers@xtra.co.nz> wrote:

> The directives EQU and CBLOCK appear to me to do the same job,
> What is the difference and in what circumstance would each be used?

Seth:

You're correct; they do the same thing.

CBLOCK is just an easier and less error-prone way to assign a series
of symbols to sequentially-numbered values.  With CBLOCK, for
instance, you can assign a bunch of labels to file registers thusly:


    CBLOCK 0x20

        TEMP
        TEMP1
        FLAGS
        BIGNUM,2
        REALBIG,4
        OUTPUT

    ENDC

With EQU, you'd write:

    TEMP    EQU     0x20
    TEMP1   EQU     0x21
    FLAGS   EQU     0x22
    BIGNUM  EQU     0x23
    REALBIG EQU     0x25
    OUTPUT  EQU     0x29

After the first time that you need to insert a label in the middle of
a series like that, you'd probably get clever and write:

    TEMP    EQU     0x20
    TEMP1   EQU     TEMP+1
    FLAGS   EQU     TEMP1+1
    BIGNUM  EQU     FLAGS+1
    REALBIG EQU     BIGNUM+2
    OUTPUT  EQU     REALBIG+4

but that still isn't anywhere near as easy as CBLOCK.

CBLOCK also allows easier code reuse, since a CBLOCK/ENDC block
without a starting constant implies a starting constant one greater
than the last one in the previous CBLOCK/ENDC block.  For example,
you can have a routine that includes both code and a CBLOCK/ENDC
block... Something like this:

    CBLOCK
        FOOREG1
        FOOREG2
    ENDC

    FOO:
        NOP        ;Do something that uses FOOREG1 and FOOREG2.
        RETURN

Then, if you cut-and-paste (or "include") that function into another
program that already has a CBLOCK, FOOREG1 and FOOREG2 will
automatically be assigned to the next-available file registers.  For
example:

    CBLOCK 0x20
        TEMP
        TEMP1
        FLAGS
        BIGNUM,2
        REALBIG,4
        OUTPUT
    ENDC

    CBLOCK
        FOOREG1        ;FOOREG1 IS ASSIGNED TO REGISTER 0x2A
        FOOREG2        ;FOOREG2 IS ASSIGNED TO REGISTER 0x2B
    ENDC

    FOO:
        NOP        ;Do something that uses FOOREG1 and FOOREG2.
        RETURN

It's not foolproof, of course -- you still should check your LST file
to ensure that your not trying to use non-existent file registers --
but it does make things a LOT easier.

And... Philosophically speaking, I think it's best to specify only as
much as is necessary, and to deliberately generalize as much of the
rest as possible.  For special-function registers like PORTA and
INTCON, I'd use EQU because it's important that those labels be
assigned to specific register numbers.  For my own registers like
TEMP and FLAGS, which can be located anywhere, I use CBLOCK as a way
of documenting the fact that it's not important that they be located
at specific locations.

-Andy


=== Andrew Warren --- aiw@cypress.com
=== IPD Systems Engineering, CYSD
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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