64 Bit Instruction



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64 Bit Instruction

Postby jaggz » Thu Aug 02, 2012 4:50 pm

Dear All,

I am curious to know the memory allocated to the 64 bit load modules.

To be precise : for 24 bit the allocated size is 16MB
for 31 bit the allocated size is 2GB

So for 64 bit what would be totaly memory allocated for the Program specified as AMODE=64 ?

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Re: 64 Bit Instruction

 

Re: 64 Bit Instruction

Postby Akatsukami » Thu Aug 02, 2012 4:58 pm

(2^64)-1 bytes, approximately 16 exabytes.
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Re: 64 Bit Instruction

Postby Robert Sample » Thu Aug 02, 2012 5:13 pm

Minus the 2-gigabyte bar.
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Re: 64 Bit Instruction

Postby steve-myers » Thu Aug 16, 2012 6:34 am

Robert Sample wrote:Minus the 2-gigabyte bar.
I think I remember reading somewhere that virtual addresses from 2 GB to 4 GB are not used for AMODE 64 addresses, possibly to avoid confusion with 31-bit addresses that have the high order bit set.
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Re: 64 Bit Instruction

Postby Robert Sample » Thu Aug 16, 2012 7:21 am

The z/OS concepts web page http://publib.boulder.ibm.com/infocente ... ts_102.htm specifically states that 31-bit addresses access below the bar, while 64-bit addresses start at 4 GB; the area between 2 GB and 4 GB of memory does not appear to be usable and is called the bar in IBM documentation. With 16 exabytes addressable, what's a mere 2 GB? IBM's documentation also says that z/OS continues to allocate 2 GB to address spaces; only if access is made above the bar will z/OS allocate memory for 64-bit addresses and it allocates whatever is used (subject to real and auxiliary storage limits and system parameters such as MEMLIMIT, of course).
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