Address Spaces Education

© 2004 IBM Corporation

IBM Systems and Technology Group

1 z/VM CP Storage Management Education Series

Address Spaces on z/ArchitectureAn Introduction

Dan FitzGeraldFriday, October 16, 2009 (Revision 2)


© 2004 IBM Corporation10/16/09Revision 2z/VM CP Storage Management Education Series2

Outline

Concept Review

Background

Introducing the ASCE

Translation Modes and Space Types

Address Space Numbers

Additional Comments



Concept Review

In this portion of the presentation, we will introduce concepts that will come up in our discussion on address spaces

Most of this information is available from the z/Architecture Principles of Operation, Chapter 3.

We will present this information as a series of definitions. This is intended to be a fast reference/review only, so please consult the Principles of Operation or your Connections Coach if you have any questions.



Concept Review

Dynamic Address Translation (DAT) – The process via which we handle virtual memory

DAT Tables – Hierarchical set of tables used for dynamic address translation (includes segment tables and page tables)

Program Status Word (PSW) – Contains information used in the execution of the currently active program

Access Registers – A special set of 16 registers, one for each general purpose register. For use with AR Mode

AR Mode – Uses the access registers when doing base displacement addressing

Determined by bits 16 and 17 of the PSW



Concept Review

There are four different types of addresses that we will encounter in z/Architecture. They are known as “absolute”, “real”, “virtual” and “logical.” Additionally, we will hear about “effective” addresses.

Absolute Address – The address assigned to a main storage location

These are the unmodified, “actual” addresses of bytes in storage.

Real Address – Identifies a location in real storage

This is an address that we will use for an access to storage

As we will see, real addresses are converted by prefixing into absolute addresses.



Concept Review

Virtual Address – Identifies a location in virtual storage– When a virtual address is used for an access to main storage, it is

translated by means of dynamic address translation (DAT) to a real address, which is then prefixed to an absolute address.

Logical Address – Your addresses are translated within whatever mode the architecture is set to

– In z/Architecture, a specific address mode can be set.For example, your machine may be set to “real address mode”. In this case, your logical addresses will be treated as real addresses.Unless otherwise specified, the storage-operand addresses for most instructions are logical addresses.



Concept Review

Effective Address – The address which exists before any transformation by dynamic address translation or any prefixing is performed

Instruction Address – Addresses used to fetch instructions from storage



Background What is an address space and how do we use it?

Address Space – A set of virtual addresses, together with the specific transformation parameters which allow each number to be associated with a byte location in memory.

The Principles of Operation defines a “virtual address” as a consecutive sequence of integer numbers

There are multiple distinct address spaces on your machine. Therefore the “same” address can point to different locations, depending on the address space

On z/Architecture, each process runs in its own address space



Background

So, what kinds of address spaces are there and how are address spaces used?

Each guest has at least one address space Guests also have the ability to define their own address spaces. Guest-defined address spaces are known as data spaces.

There are also system address spaces, which CP has created for its own purposes

The system address space is used by CP to keep track of shared data The system execution space is the address space that CP “lives” in Virtual free address space is used for a certain kind of free storage PTRM space is used to keep track of pageable PGMBKs (a PGMBK is

used for managing pages in virtual storage)



Introducing the ASCE When a virtual address is used by a CPU to access main

storage, two things happen:1. The virtual address is converted to a real address via DAT2. This real address is converted to an absolute address through the use of

prefixing.

So now the big question: how do we switch between those different address spaces?

We use something called an ASCE, which can be conceptualized as a “key” to a given address space.

Address-Space Control Element (ASCE) – 8-byte field containing the origin and length (“designator”) of the highest-level DAT table for a specific address space.

The ASCE can be considered to point to the root of the DAT tables for an address space.




An address space's ASCE can be found for use by DAT in one of three ways:

In a control register As specified by an access register Given a “real-space” designation

When the ASCE is given by a real-space designation, then the DAT is to translate the virtual address by treating it as a real address

No DAT tables are used in this case




Since there are multiple address spaces on the system, it follows that there must be multiple ASCEs

The ASCE that DAT chooses to use at a given time is determined by the translation mode specified in the PSW.

There are four translation modes: Primary-space mode Secondary-space mode Access-register mode Home-space more




When the CPU is in primary or secondary-space mode... The CPU can translate virtual addresses belonging the primary or

secondary address spaces, respectively

When the CPU is in access-register mode... It can translate virtual addresses of up to 16 address spaces These address spaces are the primary address space and the

individual address spaces specified in up to 15 access-registers Why only 15 access registers? The assembler automatically interprets

any base reference to (general or access) register 0 as a reference to the value 0.

When the CPU is in the home-space mode... it can translate virtual addresses of the home address space




Recall that each running process has its own address space This can be one of three types which tie in with the aforementioned

translation modes: primary, secondary and home

The primary address space is the base address space for the process

The secondary address space is an additional address space to be used that isn't the primary address space

The home address space contains the principle control blocks that represent a given process. For more information, refer to Chapter 5 of Principles of Operation.




It is likely that you will spend most of your time working in CMS. Normal CMS users by default only have one address space, the primary one.

This is pointed to by the “BASE” ASCBK

Other guests (Linux, DB2, special CMS setups) may be running multiple address spaces

These additional address spaces will have ASCBKs “chained” to (pointed to by) the BASE ASCBK



Address Space Numbers

Address Space Number – Each address space may be assigned an Address Space Number, or ASN, by the Control Program

The ASN uses a two-level table structure in main storage The ASN 2nd-level entry contains information about the given address

space

If the ASN 2nd-table entry is marked valid, it contains the ASCE that defines that address space



Address Space Numbers Usually, the ASNs for the primary and secondary address

spaces are assigned positions in control registers The ASN for the primary address space can be found in bits 48-63 of

control register 4 The ASN for the secondary address space can be found in bits 48-63

of control register 3 The ASN for the home address space is not assigned a position in a

control register

In access register mode, the situation is a bit different... An AR with a value of 0 specifies a primary address space; its

associated ASCE is in control register 1 An AR with a value of 1 specifies a secondary address space; its

associated ASCE is in control register 7 An AR containing any other value designates an entry in the access

list.



Address Space Numbers Access List – A table in which each entry contains the real

address of an ASN 2nd-table entry When an AR x specifies an access list entry n, the ASN 2nd-table entry

that n points to will contain the ASCE for the address space of x



Address Space Numbers The ASN 2nd-table entry contains a sequence number

(ASTESN) The ASTESN may be used to control storage references to

its associated address space These come into play when we're in AR mode and using the

access list When the access list is used to perform a storage reference, an

ASTESN in the access list entry is compared to the ASTESN in the 2nd-table entry

These values must be equal; otherwise, an ASTE-sequence exception is recognised



Address Space Numbers The ASTESN allows an access list entry to be made

unusable if authorization is changed or the designated ASN 2nd-table entry is reassigned to a different address space

In plain English: your ASN 2nd-table entry may be reused. In which case, it would have the same address

When we reuse these entries, the ASTESN in the ASN 2nd-table entry is changed

Now consider an instruction is issued for an AR-mode address space where access register x points to access list entry n...

Let's say that n points to some 2nd-table entry that has since been reused

Now the ASTESN in n does not match the ASTESN in the 2nd-table entry; an ASTE-sequence exception is issued



Additional Comments

In CP Storage Management, we only use the Primary Address Space, the Home Address Space and AR-Mode

Additionally, you won't be using ASN translation or ASN Authorization. As such, these topics have been omitted from this presentation

More information on these can be found between pages 3-23 and 3-31 of the Principles of Operation

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