5.5.2.4System z
SLES on System z systems can run either in native mode or in LPAR. Additionally, it can run as z/VM guests, which is specific to this series. This section briefly describes these three modes and how they address and protect memory. For more detailed information about System z architecture, refer to Z/Architecture Principle of Operation, at http://publibz.boulder.ibm.com/epubs/pdf/dz9zr002.pdf, or System z hardware documents at
5.5.2.4.1Native hardware mode
SLES runs directly on System z hardware in native hardware mode. Only one instantiation of SLES can run at one time. All CPUs, memory, and devices are directly under the control of the SLES operating system. Native Hardware mode is useful when a single server requires a large amount of memory. Native Hardware mode is not very common, because it requires device driver support in SLES for all attached devices, and Native Hardware does not provide the flexibility of the other two modes.
5.5.2.4.2LPAR mode
In LPAR mode, System z hardware is partitioned into up to thirty different partitions. The partitioned hardware is under the control of a hypervisor called the control program. Each partition is allocated a certain number of CPUs and a certain amount of memory. Devices can be dedicated to a particular partition, or they can be shared among several partitions. The control program provides preemptive time slicing between the partitions sharing a processor. It also ensures that each partition gets the exact allocated share of the CPU, even if the remainder of the processor is unused. SLES runs in one of these logical partitions. LPAR mode provides more flexibility than Native Hardware mode, but still requires device driver support for devices dedicated to a partition.
5.5.2.4.3z/VM Guest mode
In z/VM Guest mode, SLES runs as a guest operating system on one or more z/VM virtual machines. z/VM virtualizes the hardware by providing the same interface definition provided by the real hardware to a guest operating system. Guests operate independently of each other, even though they share memory, processors, and devices. The z/VM Guest mode provides even more flexibility than LPAR mode because, unlike LPAR, z/VM virtual machines allow dynamic addition or deletion of memory and devices. The z/VM Guest mode is the most commonly deployed mode because of the flexibility that it provides.
In terms of memory addressing, all three modes believe they are operating directly on the System z hardware. The control program (either LPAR or VM, or both) sets up their paging tables and zoning array so the Start Interpretive Execution (SIE) instruction can do the address conversion. The control program does not actively convert any addresses.
5.5.2.4.4Address types
z/Architecture defines four types of memory addresses: virtual, real, absolute, and effective. These memory addresses are distinguished on the basis of the transformations that are applied to the address during a memory access.
Virtual address: A virtual address identifies a location in virtual memory. When a virtual address is used to access main memory, it is translated by a Dynamic Address Translation (DAT) mechanism to a real address, which in turn is translated by prefixing to an absolute address. The absolute address of a virtualized system is, in turn, subjected to dynamic address translation in VM or to zoning in LPAR.
Real address: A real address identifies a location in real memory. When a real address is used to access main memory, it is converted by, prefixing mechanism, to an absolute address.
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