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 Post subject: 68000 initial code
PostPosted: Mon Dec 27, 2004 6:49 pm 

Joined: Mon Dec 27, 2004 6:11 am
Posts: 1
Location: Lehigh Acres, florida
Firstly a very sincere thank you for this great work that you have contributed to the community of fellow programmers and students.
I am trying to figure out how the start up code and the hardware implementation of it when you use the following trick.
I know, some designers, use a very small rom to provide the inital stack pointer and the address of the program counter. Then the rom is taken out of the memory space leaving all the initial memory as ram.
My question relates is to how they do the manipulation?
Do they have 2 roms, one with the initial code to provide the information and a second one in high memory where the jump is made to.??
Is it only one rom?
Thanks in advance, J. DeMOya :roll:


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PostPosted: Mon Dec 27, 2004 10:21 pm 
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Joined: Thu Dec 16, 2004 6:42 pm
Posts: 1118
Yes to all of the above.

With proper decoding of Address and R/W lines it is posible to map a ROM or RAM to multiple address spaces. That would allow you to use part of it as Reset code and part as program code that would run in a different address space.

In the industrial systems that I programmed we used a combination of Battery Backed CMOS RAM, and RAM. The CMOS RAM held the Reset code and program. The CMOS RAM was write protected by the MMU.

Prof. Kelly

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