Technical data

SDA Description

8005D1C8 UCB$L_SVAPTE 80062720

8005D1CC UCB$W_BOFF 0795

8005D1CE UCB$W_BCNT 006D

8005D1D0 UCB$B_ERTCNT 00

8005D1D1 UCB$B_ERTMAX 00

8005D1D2 UCB$W_ERRCNT 0000

SDA>

If you have only one printer in your system conﬁguration, you do not need to use

the FORMAT command. Instead, you can use the command SHOW DEVICE LP.

Because only one printer is connected to the processor, only one UCB is associated

with a printer for SDA to display.

The output produced by the FORMAT @R5 command shows that UCB$W_BOFF

contains a value greater than that in UCB$W_BCNT; it should be smaller.

Therefore, the value stored in BOFF is incorrect.

Thus, the value of BOFF is not the number of characters that remain in the

buffer. This value is used in calculating an address that is referenced at an

elevated IPL. When this address is within a null page (unreadable in all access

modes), an attempt to reference it causes the system to fail.

9.4.3 Identifying and Correcting the Defective Code

Examine the printer driver code to locate all instructions that modify UCB$W_

BOFF. The value changes in two circumstances:

• Immediately after the driver detects that the printer is not ready and that

the problem is not a paper problem (line 609).

• When the wait-for-interrupt routine’s timeout count of 12 seconds is

exhausted (lines 616 and 630). At this time, the contents of R1, plus 1,

are stored in UCB$W_BOFF (line 631).

When the printer times out, the driver should not modify UCB$W_BOFF. It does

so, however, in line 631. The driver should modify the contents of UCB$W_BOFF

only when it is certain that the printer printed the character. When the printer

times out, this is not the case. Furthermore, the wait-for-interrupt routine

preserves only registers R3, R4, and R5, so that only those registers can be used

unmodiﬁed after the execution of the wait-for-interrupt routine. Thus, the use of

R1 in line 631 is an error.

To correct the problem, change the WFIKPCH argument (line 616) so that, when

the printer times out, the WFIKPCH macro transfers control to 50$ rather than

to 40$.

SDA–30