Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!amdahl!dwl10
From: dwl10@amdahl.uts.amdahl.com (Dave Lowrey)
Newsgroups: comp.sys.cbm
Subject: 1541 coding question
Message-ID: <24611@amdahl.uts.amdahl.com>
Date: 14 Mar 88 17:17:07 GMT
Organization: Amdahl Corporation, Houston TX
Lines: 42
Keywords: 1541 rom

I have a question regarding 1541 programming. I have been studying the ROM
listings, and examples of programming the 1541 yourself, and have come
upon an interesting bit of code that I don't fully understand.
 
The scenerio is that we are in "Executed" code (JOB code 0xe0), and have
JSR'ed to a subroutine that finds a header for a specific sector.
Upon return from the subroutine, the following code is executed:
(Note...addresses are not actual addresses from the ROM)
 
c100  CLV
c101  BVC  $c101
   ..
   ..
   ..
 
I assume that the disk controller interrupts the 6502 when "something"
happens, and the IRQ/NMI interrupt handeler sets the Overflow flag, so
our "endless" loop ends.
 
My question is: exactly why/when is the oveflow flag set, and who is
setting it?
 
There are several occurances of this code in the ROM. I also have
sample programs from a book that do this, with the comments such as
"Wait for header" and "wait for data". I would like specific info
on what is happening here.
 
Thanks.....
 

-- 
-------------------------------------------------------------------

                  "Familiarity breeds attempt"

                          Dave Lowrey
                          Amdahl Corp.
                          Houston, Texas
                          (713)-850-8828
                         ...!{ihnp4,cbosgd,hplabs,oliveb}!amdahl!dwl10

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