IBM System/370 145 Manual page 200

EXPANDED MACHINE CHECK FACILITIES
Implementation of the machine check class of interruption on the
System/370 Model 145 has been expanded in order to enhance error
recording and error recovery procedures. Programming support of the
extended machine check interruption is provided by the Model 145 MCH and
MCAR routines of OS and DOS, respectively.
The machine check interruption facilities of the Model 145 differ
from those of Models 30 and 40 as follows:
• Five types of machine check are defined.
• A machine check interruption occurs to permit the recording of
errors corrected by the hardware as well as to allow recovery
routines to hand1e errors that cannot be corrected by hardware.
• Machine check interruption masking is expanded to handle
selective disabling and enabling of the CPU for the interruption
types defined.
• The size of "the fixed area in lower processor storage is
increased to accommodate the storing of additional machine status
and diagnostic information when a machine check interruption
occurs.
• Error conditions are defined that cause the Model 145
to stop functioning immediately because the nature of the machine
malfunction prevents valid processing from continuing.
The Model 145 presents one of five types of machine check
interruption conditions, depending on the specific machine malfunction,
and each type of interruption is maskable. Machine check interruption
conditions are either repressible or exigent (formerly called soft or
hard).
A repressible machine check condition exists after the hardware
has been successful in correcting an error or after an error has
occurred that does not prevent continued successful execution of
instructions. An interruption can occur after a repressible machine
check condition so that the failure can be recorded. System operation
continues after the error is logged. For example, if an error occurs
during the execution of an instruction and if the microinstruction retry
hardware corrects the error by reexecuting the failing instruction, a
repressible machine check condition exists at the completion of the
successful execution.
An exigent machine check condition exists when hardware retry fails
or is not possible. Exigent conditions are those that prevent
successful execution of the current instruction. For example, if the
microinstruction retry hardware has not corrected a failing instruction
after eight retries, an exigent rather than a repressible machine check
condition exists after the last unsuccessful retry,.
Figure 50.10.3 shows the layout of the model-dependent fixed
processor storage in the Model 145. The total fixed storage area
consists of four areas: the fixed locations in decimal addresses 0-159
(shown in Figure 10.10.2), the I/O communications area in locations 160-
191 (used during EC mode I/O interruptions), the fixed logout area in
locations 216-511, and the CPU extended logout area of 192 bytes, which
begins at location 512 and continues to location 703 unless the logout
pointer is changed by programming. The layout of the Model 145 fixed
locations from address 160 to 703 is the same in BC and EC modes. The
I/O address is stored in locations 185 to 187 only in EC mode, however.
A
logout to the System/370 model-dependent fixed logout area (216-
511) occurs when any type of machine check interruption is "taken. The
190 A Guide to the IBM System/370 Model 145