International Business Machines Corporation

Richard Eugene Birney

Robert Allen Hood

Michael Ian Davis

William Steese Osborne

Lynn Allan Graybiel

Samuel Kahn

Joel C Leininger

George P Taylor

Thomas Stephen McDermott

Larry Edward Wise

A carry save adder (CSA) is adapted for use in dividing operations by providing it with a lookahead capability whereby it accurately predicts whether or not each of the proposed complemental subtractions in a division process could be successfully performed if actually attempted, and it actually performs only those subtractions that will not result in overdrafts. Each bit position of the carry save adder is arranged to receive sum, carry and data inputs and to furnish sum, carry, presum and precarry outputs. The sum and carry output bits are latched and remain undisturbed until the next complemental subtraction is performed. The presum and precarry bits are not latched and are fed into a lookahead logic network which analyzes the presum and precarry bit patterns derived from all of the CSA bit positions to determine rapidly by a logical trial procedure whether the divisor could be subtracted from the dividend or partial remainder value currently registered in the sum and carry latches of the CSA without causing an overdraft. While this trial procedure is in progress, the presum and precarry values may change without altering the latched sum and carry values. If the trial procedure indicates that a proposed complemental subtraction would be unsuccessful, then such subtraction will be skipped without actually being performed by the CSA, thus eliminating the necessity of determining whether overdrafts have occurred and correcting the overdrafts.

Divider using carry save adder with nonperforming lookahead

The disclosure describes equate operand spaces (EOS) control over the addressabilities accessed by means of different address keys in an address key register (AKR) in a processor. Executing instructions, and their source and sink type operands may have different address keys in the AKR, and therefore different addressabilities. When enabled, the EOS control forces each source operand fetch to occur within the sink operand addressability specified in the AKR, even though the AKR explicitly contains a different addressability for source operands. When the EOS feature is disabled, the source operand addressability contained in the AKR is used when fetching source operands. An EOS field in the AKR stores whether the EOS state is enabled or disabled in the processor.

Equate operand address space control system

The disclosure describes instruction operated controls for loading or storing address key values into or from one or more address key register sections in a key-register-controlled addressing system. The controls load or store one or all key register sections of an address key register (AKR) in a processor from or to a word in either a main memory or a general purpose register (GPR). Both the load or store controls are operated by the same instruction format, in which one field indicates whether the operation is to be a load or store of the designated AKR section(s). Another field designates one AKR section, or all AKR sections, which are to be loaded or stored. A still further field designates whether the operation is to be from or to main memory or a GPR. The disclosure provides circuits which operate with microcode to perform these operations.

Address key register load/store instruction system

Combines a storage protect key stack with an access key register (AKR) and active access key (AAK) select circuits. Storage key entries in the stack correspond to the physical blocks in main memory. This combination can provide storage protection for different storage access types within address sub-ranges in the main memory associated with respective access keys. The sub-ranges are blocks of addresses within the full range of addresses of the physical memory. The protect key operation applies to physical addresses, and it obtains system addressing compatibility with an address translation operation using the same access keys as address keys with program logical addresses. Special features include a shared protect key, which need not be loaded in the AKR, to make specified sub-range(s) shareable by all users of the system, so that any user can access the blocks in memory associated with the shared protect key. For I/O accesses, an override is provided which ignores any read-only control of any memory block to which an I/O access is requested. Supervisor accesses can be made in all key areas, regardless of the AAK, the protect keys, or the read-only flag bits.

Non-translatable storage protection control system

Active address keys (AAK) are translated into respective addressabilities in physical main memory. Each addressability comprises one or more physical blocks which may be scattered in the main memory. Each address key represents the assigned addressability in main memory for a logical address space. Plural key register sections may be loaded with the same or different address keys. For each storage access request, received from a processor or I/O channel, AAK select circuits outgate to the translator the key in the key register section corresponding to the type of the current storage access request to determine the addressability available to the access request. Each address key identifies a particular stack of one or more segmentation registers. Each segmentation register can be assigned the address of any segment (i.e. a block of contiguous physical addresses located anywhere in the main memory). Each segmentation register also has validity and read-only flag bits for its assigned block. Each stack can translate a contiguous set of logical addresses into physical addresses. For each storage access request for a logical address, and not apparent to the usury program, a stack is addressed by the AAK to translate the logical address to a real address within the assigned addressability. Then a register in the stack is selected by high-order bits in the logical address. The addressed register outputs an assigned block address. Low order bits in the logical address select a byte address in the assigned block. The main memory can have any physical size, which is coordinated with the number of stacks, and to the number of segmentation registers in each stack.

Key controlled address relocation translation system

A plurality of key register sections in a processor respectively associated with different machine-sensed types of accesses to a main storage of a computer system. A processor address key register (AKR) includes the following sections: (1) a section associated with an instruction-fetch type access, (2) a section associated with a source-operand fetch type access, and (3) a section associated with a sink-operand store/fetch type access. Other key register sections may be associated with respective sub-channel store/fetch type accesses. Circuits are provided which sense the different access types to select and outgate a key contained in the corresponding key register section. The values of the keys are associated with different addressabilities (i.e. address spaces). Each different key value is associated with a different stack of translation registers for containing the block addresses in real storage currently assigned to the respective addressabilities. Each key outgated from a respective register section selects an address space by selecting the associated stack of addressing registers to relocatably translate a logical address to a physical storage location within the address space selected by the outgated key.

Key register controlled accessing system

A control circuit arrangement for storing the addressability defined by the current active address key (AAK) being accessed in an address key register (AKR) in a processor. This AAK is stored in a last AAK register. When a hard or soft check interrupt occurs, the AAK stored in the last AAK register is designated as the processor's last key saved (i.e. LKSA) to define an interrupted addressability being used by the processor at the time of an interrupt. Upon occurrence of an interrupt, the LSKA represents the interrupted addressability, which is then made available to the processor by gating the LKSA into a source operand key section in the AKR from the processor's last AAK register, and setting the key for a supervisor program into another section of the AKR, so that the supervisor program can take corrective or termination actions. Until the LKSA gating into the AKR is completed, no AAK can be ingated into the last AAK register. After this in gating to the AKR is completed, the ingating disablement to the last AAK register is released.

System for controlling address keys under interrupt conditions

System mode controls for obtaining limited addressability for supervisor programming operations without disturbing a user address key currently contained in a user key register (UKR). The mode controls are provided by bits in a system register called a level status register (LSR), which include an APM bit, and a supervisor state bit. The largest supervisor addressability is obtained when both the APM and supervisor state bits are set on, which permits execution of a supervisor program which can access user data and programs. In more detail, each instruction fetch must be in the supervisor key area, identified by a predetermined supervisor key value which is not in the UKR, while each operand of the fetched supervisor instruction is accessed in the user key area identified by the current key in the UKR. The supervisor is not permitted to access any user area which does not have its key in the UKR. Thus, the supervisor can be prevented from having addressability over part or all of the main memory. However, if the APM bit is off while the supervisor bit is on, all instruction and operand storage accesses can only be made in the supervisor key area, regardless of whether the supervisor key or user key is in the UKR. Hence no user area is accessible to the supervisor. But, if the supervisor bit is off, all instruction and operand accesses can only be made in the user area of the key in the UKR. Hence the supervisor programs cannot execute.

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Boca Raton, FL, United States of America

Richard Eugene Birney