DonNews #07 23 апреля 1999

Программирование

Coding - Definition of a page number of upper memory.

Snake \ OHG


                 Coding.


   Coding, it's certainly loud skazano.Zdes
will be just one small files of,
which, I think anyone can
useful.

   When I wrote a small prog for
cache, whereby when you click on
Magic could start the boot, or enter
in the STS (first I wanted to in this section
put it, but then changed his mind, but if
someone wants, then the next number
You can place and her). When entering the STS
saved all the registers, the address of the interrupted
program address stack, but the page number
upper memory to take it nowhere.
Therefore it was necessary to determine its software.


   That's listing of this program:

FPG LD DE, (# C000)

        LD BC, # 7FFD

        LD # C000, BC

        LD A, # D7
FPG1 INC A

        BIT 3, A

        JR Z, FPG2

        ADD A, # 38
FPG2 OUT A

        LD HL, (# C000)

        AND A

        SBC HL, BC

        JR NZ, FPG1

        LD # C000, DE

        ...


   A little explanation: the register DE
stored 2 bytes from address # C000, in
Register A page number from which
search begins, # D7 - to the first
the addition of a # 10 (so the program
takes up less than 2 bytes). At
# C000 recorded two bytes # FD, # 7F, by
which we shall seek the desired page.
Then we begin in order to switch
page and compare the two bytes at
# C000 with the contents of register BC, as soon as
all coincide, then the page is found. On
output in register A will be in room
page in which was launched this
program.

   The program takes only 34 bytes and
is designed to 512kb with an extension for two
Senior bits port # 7FFD.Stroka ADD A, # 38
is designed to reset the third
bit, and increase by 1 two bits of the
Register A, which will switch
additional memory.

   Naturally, the 128y car program
be it the same work just fine.

   Disadvantages: All the same can be obtained
so that bytes # FD, # 7F, which is
search may be in another page.
Then, if its number is smaller, the definition of
going wrong, and in the page
you had to find a spoiled 2
bytes.

   And of course, does not define what
was included screen.


   P.S: OUT A = OUT (C), A

        LD # C000, BC = LD (# C000), BC

        XAS = RULEZ.