Английская Википедия:IBM System/370-XA

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IBM S/370-XA registers

General Registers 0-15 Two's complement value 0 31

Control Registers 0-15 See Principles of OperationШаблон:Rp 0 31

Floating Point Registers 0-6 S Biased exponent Mantissa 0 1 7 8 31 Mantissa (continued) 32 63

Extended Architecture Extended Control mode PSWШаблон:Rp 0 R 0 0 0 T I O E X Key 1 M W P S 0 CC Program Mask 0 0 0 0 0 0 0 0 0 1 2 4 5 6 7 8 11 12 13 14 15 16 17 18 19 20 23 24 31 A Instruction Address 32 33 63 Шаблон:Nowrap Bits Field Meaning 1 R PER Mask 5 T DAT mode 6 IO I/O Mask; subject to channel mask in CR2 7 EX External Mask; subject to external subclass mask in CR0 8-11 Key PSW key 12 E=1 Extended Control mode 13 M Machine-check mask 14 W Wait state 15 P Problem state 16 S Address-Space Control 0=primary-space mode 1=Secondary-space mode 18-19 CC Condition Code 20-23 PM Шаблон:Nowrap Bit Meaning 20 Fixed-point overflow 21 Decimal overflow 22 Exponent underflow 23 Significance 32 A Addressing mode 0=24 bit; 1=31 bit 33-63 IA Instruction Address IBM documentation numbers the bits from high order to low order; the most significant (leftmost) bit is designated as bit number 0.

IBM System/370-XA is an instruction set architecture introduced by IBM in 1983 with the IBM 308X processors. It extends the IBM System/370 architecture to support 31-bit virtual and physical addresses, and includes a redesigned I/O architecture.

31-bit virtual addressing

In the System/360, other than the 360/67, and System/370 architectures, the general-purpose registers were 32 bits wide, the machine did 32-bit arithmetic operations, and addresses were always stored in 32-bit words, so the architecture was considered 32-bit, but the machines ignored the top 8 bits of the address resulting in 24-bit addressing. Much of System/360's and System/370's large installed code base relied on a 24-bit logical address; In particular, a heavily used machine instruction, Шаблон:Code, Load Address, explicitly cleared the top eight bits of the address being placed in a register. If the 24-bit limit were to be removed, this would create migration problems for existing software.

This was addressed by adding an addressing mode bit to the Program Status Word controlling whether the program runs in 24-bit mode, in which the top eight bits of virtual addresses are ignored, or 31-bit mode, in which only the uppermost bit of virtual addresses are ignored.Шаблон:RpШаблон:Rp Several reasons were given for the choice of 31 bits instead of 32 bits:

1. The desire to retain the high-order bit as a "control or escape bit."Шаблон:Rp In particular, the standard subroutine calling convention marked the final parameter word by setting its high bit.
2. Interaction between 32-bit addresses and two loop control instructions, Шаблон:Code and Шаблон:Code that treated their arguments as signed numbers when doing comparisons (and which was said to be the reason TSS used 31-bit addressing on the Model 67).Шаблон:Rp
3. Input from key initial Model 67 sites, which had debated the alternatives during the initial system design period, and had recommended 31 bits (instead of the 32-bit design that was ultimately chosen at the time).Шаблон:Rp

Certain machine instructions in this 31-bit addressing mode alter the addressing mode bit. For example, the original subroutine call instructions Шаблон:Code, Branch and Link, and its register-register equivalent, Шаблон:Code, Branch and Link Register, store certain status information, the instruction length code,Шаблон:Efn the condition code and the program mask, in the top byte of the return address. A Шаблон:Code, Branch and Save, instruction was added to allow 31-bit return addresses. Шаблон:Code, and its register-register equivalent, Шаблон:Code, Branch and Save Register, was part of the instruction set of the 360/67, which was the only System/360 model to allow addresses longer than 24 bits. These instructions were maintained, but were modified and extended for 31-bit addressing.

Additional instructions in support of allowing calls between 24-bit-addressing and 31-bit-addressing code include two new register-register call/return instructions which also effect an addressing mode change, Шаблон:Code, Branch and Save and Set Mode, the 24/31 bit version of a call where the linkage address including the mode is saved and a branch is taken to an address in a possibly different mode, and Шаблон:Code, Branch and Set Mode, the 24/31 bit version of a return, where the return is directly to the previously saved linkage address and in its previous mode. Taken together, Шаблон:Code and Шаблон:Code allow 24-bit calls to 31-bit (and return to 24-bit), 31-bit calls to 24-bit (and return to 31-bit), 24-bit calls to 24-bit (and return to 24-bit) and 31-bit calls to 31-bit (and return to 31-bit).Шаблон:Rp

Like Шаблон:Code (the 24-bit-only form of a call), Шаблон:Code is used as Шаблон:Code, where the linkage address and mode are saved in register 14, and a branch is taken to the subroutine address and mode specified in register 15. Somewhat similarly to Шаблон:Code (the 24-bit-only form of an unconditional return), Шаблон:Code is used as Шаблон:Code, where 0 indicates that the current mode is not saved (the program is leaving the subroutine, anyway), and a return to the caller at the address and mode specified in register 14 is to be taken.

31-bit physical addressing

System/370 initially supported only 24-bit physical addresses; the extended real address feature extended this to 26-bit addresses.

System/370-XA changed the page table entry format to support 19 bits of page frame address; pages are 4 KB in 370-XA, so combining a 19-bit page frame address with a 12-bit offset within the page produces a 31-bit physical address.Шаблон:Rp Channel command words can be in one of two formats, with format 0 being the System/370 format, with a 24-bit data address, and format 1 being an additional format, with a 31-bit data address.Шаблон:RpШаблон:Rp

I/O

Шаблон:Expand section System/370-XA introduced a channel subsystem that performed I/O queuing previously done by the operating system.