360 Assembly/Privileged Instructions

Privileged Instructions are programming instructions that need to be restrited to the operating system in order to protect system resources, system integrity, or system security. These include, but are not limited to:

• I/O instructions. To make sure two programs do not try to write to a printer, or edit a database record, or other errors that could corrupt files, a program issues I/O requests to the operating system, and it handles these operations.
• Load PSW. The PSW (program status word) contains the status about the computer and the currently executing job.
• Insert storage key, set storage key. Change or read any memory.
• Page table functions. Contaminate other jobs, lose pages swapped out to disc, corrupt memory and/or other jobs.
• Diagnose. Used by operating systems to issue requests to the hypervisor.

Programs that do not have privileges are known as "problem" programs. Without protecting the PSW, page tables, or I/O devices, problem programs could:

• Grant themselves privileges
• Allow themselves to reset their storage key, allowing them to change any memory
• Rewrite, contaminate or destroy page tables, losing or corrupting other jobs
• Change the system date and time
• Halt the machine

Anyone having heard about or seen the effects of malware or ransomware can understand the kind of damage a rogue privileged program do to a computer. Unless you have your own machine (like using the Hercules emulator on Linux, Mac, Raspberry Pi, or Windows, and don't care if something in the virtual environment is damaged), or you know exactly what you're doing, it's probably best to restrict privileged functions to the operating system than allow just anyone to use them willy-nilly. However, because sometimes problem programs in a production environment legitimately have a need to execute privileged instructions or access operating system tables or memory belonging to another process, many installations will install a "private" SVC routine ("private" in that is only set up on that machine, and is not part of the "stock" operating system supplied by the developer) and that SVC either grants privileged state to that process or changes an operating system table to grant the program additional privilege(s). Often the same SVC can be used to reset the request, revoking the privilege.

These are the privileged instructions: Legend:

•         Available on all models unless indicated by (360 only) or (360 & 370 only)
•         Available on the 370, 390 and z/System
•         Available on the 390 and z/System
•         Available only on the z/System

Using any of these in a problem (unprivileged) program will cause a protection exception. Using one of these instructions in a privileged program on the wrong class of machine will cause an operation exception.

Opcode (Hex)	Mnemonic	Description
08	SSK	Set Storage Key (360 only)
09	ISK	Insert Storage Key (360 only)
82	LPSW	Load Program Status Word
84	WRD	Write Direct (360 only)
85	RDD	Read Direct (360 Only)
9C	SIO	Start I/O (360 Only)
9D	TIO	Test I/O (360 Only)
9D01	CLRIO	CLeaR I/O (370 only)
9E	HIO	Halt I/O (360 only}
9F	TCH	Test CHannel (360 & 370 only)
B200	CONCS	Connect Channel Set
B201	DISCS	DISconnect Channel Set
B202	STIDP	Store CPU ID
B204	SCK	Set Clock
B206	SCKC	Set Clock Comparator
B207	STCKC	Store Clock Comparator
B208	SPT	Set CPU Timer
B209	STPT	Store CPU Timer
B20A	SPKA	Set PSW Key From Address
B20B	IPK	Insert PSW Key
B20D	PTLB	Purge TLB
B210	SPX	Set Prefix
B211	STPX	Store Prefix
B212	STAP	Store CPU Address
B213	RRB	Reset Reference Bit
B221	IPTE	Invalidate Page Table Entry
B223	IVSK	Insert Virtual Storage Key
B224	IAC	Insert Address Space Control
B225	SSAR	Set Secondary ASN
B226	EPAR	Extract Primary ASN
B227	ESAR	Extract Secondary ASN
B228	PT	Program Transfer
B229	ISKE	Insert Storage Key Extended
B22A	RRBE	Reset Storage Key Extended
B22B	SSKE	Set Storage Key Extended
B22C	TB	Test Block
B22E	PGIN	Page In
B22F	PGOUT	Page Out
B230	CSCH	Clear SubCHannel
B231	HSCH	Halt SubCHannel
B232	MSCH	Modify SubCHannel
B233	SSCH	Start SubCHannel
B234	STSCH	Store SubCHannel
B235	TSCH	Test SubCHannel
B236	TPI	Test Pending Interruption
B237	SAL	Set Address Limit
B238	RSCH	Resume SubCHannel
B239	STCRW	Store Channel Report Word
B23A	STCPS	Store Channel Path Status
B23B	RCHP	Reset Channel Path
B23C	SCHM	Set Channel Monitor
B246	STURA	STore Using Real Address
B248	PALB	Purge ALB
B24B	LURA	Load Using Real Address
B254	MVPG	Move Page
B277	RP	Resume Program
B2B1	STFL	Store Facility List
B2B2	LPSWE	Load PSW Extended
B6	STCTL	Store ConTroL
B7	LCTL	Load ConTroL
B905	LURAG	Load Using Real Address (64 bit)
B925	STURG	Store Using Real Address
B928	PCKMO	Perform Cryptographic Key Mgmt Operations
B98E	IDTE	Invalidate DAT Table Entry
B98F	CRDTE	Compare and Replace DAT Table Entry
B99A	EPAIR	Extract Primary ASN and Instance
B99B	ESAIR	Extract Secondary ASN and Instance
B99D	ESEA	Extract and Set Extended Authority
B99E	PTI	Program Transfer with Instance
B99F	SSAIR	Set Secondary ASN and Instance
B9A1	TPEI	Test Pending External Interruption
B9A2	PTF	Perform Topology Function
B9AA	LPTEA	Load Page Table Entry Address
B9AE	RRBM	Reset Reference Bits Multiple
B9AF	PFMF	Perform Frame Management Function
E500	LASP	Load Address Space Parameters
E501	TPROT	Test PROTection
E502	STRAG	STore Real Address Grande
EB0F	TRACG	TRACe Grande
EB25	STCTG	STore ConTrol (64)
EB2F	LCTLG	Load Control (64)