Error in explanation of the source code?

From: Baltissen, GJPAA (Ruud) <>
Date: Wed, 12 May 2010 11:31:47 +0200
Message-ID: <>
Hallo allemaal,

I'm busy with creating the source code of the 8250 using the German book
I uploaded a short while ago and the schematics found on Bo's site.

            ;**  Check number of heads
A2 02                       ldx     #$02

A5 82                       lda     OREGB__
29 40                       and     #$40
F0 01                       beq     D_0571

CA                          dex
            D_0571                                  ;
8E AC 04                    stx     NSIDES

The above part checks the number of heads, according the book. The line
'and #$40' checks PB6 of the 6530 which is unconnected, thus reads as a
(H) and therefore X is decremented, resulting in only one head. Or am I
wrong? (I hope so)

            ;**  Check type of drive, 2A or 2C
A2 01                       ldx     #$01

A5 82                       lda     OREGB__
29 10                       and     #$10            ; check if jumper
has been placed
D0 01                       bne     D_057D          ; no, -> 8050

CA                          dex
            D_057D                                  ;
8E EA 04                    stx     VERSWC_
D0 03                       bne     D_0585
A2 07                       ldx     #$07
2C                          .by $2C                 ; dummy BIT opcode
            D_0585                                  ;
A2 17                       ldx     #$17
86 83                       stx     OREGDB_ 

The book mentions that variable VERSWC_ ($04EA) on the disk controller
is used to tell the drive to use either 2A or 2C as format code. But 2A
is only meant to be used by the 2031/4040/1541 AFAIK. Both 8050 and 8250
disks use 2C.
But at D_0582 the direction bit for the head select is set according the
found result; dis- or enabling the use of this bit. So IMHO _this_ part
checks the number of heads, not the above one.

Then there is the variable VERNUM ($109F) for the Bus Controller. Here
is either 'A' or 'C' stored, depending whether is is an 8050 or 8250.
And I found a place where it is used to fill place $02 of the BAM. Now
you understand why I'm more then a bit confused.

Any comment is welcome!

    / __|__
   / /  |_/     Groetjes, Ruud
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    \___|       URL:


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Received on 2010-05-12 10:00:03

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