ADDRESS SPACES - IBM Mainframe

Main storage consists of millions of individual storage locations, each of which can store one character, or byte, of information. To refer to a particular location, you use an address that indicates the storage location's offset from the beginning of memory. The first byte of storage is at address 0, the second byte is at address 1, and so on. Each successive byte of main storage has an address that is one greater than the previous byte of storage.

An address space is simply the complete range of addresses—and, as a result, the number of storage locations—that can be accessed by the computer. The maximum size of a computer's address space is limited by the number of digits that can be used to represent an address. To illustrate, suppose a computer records its addresses using six decimal digits, such a computer could access storage with addresses from 0 to 999,999. So the computer's address space could contain a maximum of one million bytes of storage. (Of course, computers actually record their addresses using binary digits rather than decimal digits.)

The original System/370 processors used 24-bit binary numbers to represent addresses (a bit is the binary equivalent of a digit). Since the largest number that can be represented in 24 bits is about 16 million 'abbreviated 16M), an address space on a System/370 cannot contain more than 16M bytes of storage. Because this 16MB address space limitation severely restricted the capabilities of the System/370, IBM replaced it in the early 1980s with a new architecture known as 370-XA. 370-XA processors can operate in 370 mode using standard 24bit addresses or in XA, or Extended Architecture, mode using 31-bit addresses. In XA mode, the largest address that can be represented—and therefore the largest address space that can be used—is about 2 billion, or 20.ADDRESS SPACES

Main storage consists of millions of individual storage locations, each of which can store one character, or byte, of information. To refer to a particular location, you use an address that indicates the storage location's offset from the beginning of memory. The first byte of storage is at address 0, the second byte is at address 1, and so on. Each successive byte of main storage has an address that is one greater than the previous byte of storage.

An address space is simply the complete range of addresses—and, as a result, ADDRESS SPACES

Main storage consists of millions of individual storage locations, each of which can store one character, or byte, of information. To refer to a particular location, you use an address that indicates the storage location's offset from the beginning of memory. The first byte of storage is at address 0, the second byte is at address 1, and so on. Each successive byte of main storage has an address that is one greater than the previous byte of storage.

An address space is simply the complete range of addresses—and, as a result, the number of storage locations—that can be accessed by the computer. The maximum size of a computer's address space is limited by the number of digits that can be used to represent an address. To illustrate, suppose a computer records its addresses using six decimal digits, such a computer could access storage with addresses from 0 to 999,999. So the computer's address space could contain a maximum of one million bytes of storage. (Of course, computers actually record their addresses using binary digits rather than decimal digits.)

The original System/370 processors used 24-bit binary numbers to represent addresses (a bit is the binary equivalent of a digit). Since the largest number that can be represented in 24 bits is about 16 million 'abbreviated 16M), an address space on a System/370 cannot contain more than 16M bytes of storage. Because this 16MB address space limitation severely restricted the capabilities of the System/370, IBM replaced it in the early 1980s with a new architecture known as 370-XA. 370-XA processors can operate in 370 mode using standard 24bit addresses or in XA, or Extended Architecture, mode using 31-bit addresses. In XA mode, the largest address that can be represented—and therefore the largest address space that can be used—is about 2 billion, or 20.the number of storage locations—that can be accessed by the computer. The maximum size of a computer's address space is limited by the number of digits that can be used to represent an address. To illustrate, suppose a computer records its addresses using six decimal digits, such a computer could access storage with addresses from 0 to 999,999. So the computer's address space could contain a maximum of one million bytes of storage. (Of course, computers actually record their addresses using binary digits rather than decimal digits.)

The original System/370 processors used 24-bit binary numbers to represent addresses (a bit is the binary equivalent of a digit). Since the largest number that can be represented in 24 bits is about 16 million 'abbreviated 16M), an address space on a System/370 cannot contain more than 16M bytes of storage. Because this 16MB address space limitation severely restricted the capabilities of the System/370, IBM replaced it in the early 1980s with a new architecture known as 370-XA. 370-XA processors can operate in 370 mode using standard 24bit addresses or in XA, or Extended Architecture, mode using 31-bit addresses. In XA mode, the largest address that can be represented—and therefore the largest address space that can be used—is about 2 billion, or 20.

In the late 1980s and early 1990s, IBM extended this architecture even further with the introduction first of ESA/370, then ESA/390. These designs utilize the same 31-bit addresses that 370-XA uses, but they extend the architecture in other ways. For example, ESA/390 allows the use of high-speed fiber-optic channels known as ESCON.

As you might guess, MVS/370 was designed to operate on System/370 processors that utilize 24-bit addresses, and MVS/XA was designed *o operate on 370-XA processors that use 31-bit addresses. MVS/ESA also uses 31-bit addresses, but runs only on ESA/370 and ESA/390 processors.

One way to think of virtual storage is that it lets the computer push its address space to the maximum allowed by the address format, even if the amount of real storage installed on the processor is less than the maximum. So, in 370 mode, virtual storage can simulate a 16MB address space, even if only 4MB or BMB of real storage is actually installed. That is precisely how virtual storage worked on older System/370 operating systems such as OS/VS1 and OS/VS2 SVS. Under MVS, however, the concept of virtual storage is taken one step further.


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