Control Data Cyber 170 State Hardware Reference Manual page 49

Central Memory
SECDED Logic
t h e
1.
2 .
3.
4 .
5 .
6.
7 .
The SECDED l o g i c c o r r e c t s s i n g l e - b i t e r r o r s during
a
CM r e a d , permitting
unimpeded computer operation. The SECDED l o g i c prepares f o r t h e e r r o r
c o r r e c t i o n by generating e r r o r c o r r e c t i o n code (ECC) b i t s f o r each d a t a word
and by s t o r i n g these E C C b i t s i n CM with t h e d a t a word durlng t h e CM w r i t e .
Table 2-2 l i s t s t h e hexadecimal codes f o r d l t h e combinations of syndrome b i t s
with t h e number of t h e d a t a b i t assigned t o each code o r a note c a t e g o r i z i n g
code.
During a CM r e a d , CM then performs t h e following SECDED sequence.
Read 1 CM word and generate new E C C b i t s f o r d a t a p o r t i o n of CM word.
Compare new E C C b i t s with CM word E C C b i t s .
I f old and new ECC b i t s match, no e r r o r e x i s t s .
Send d a t a t o the
r e q u e s t i n g u n i t .
If b i t s do not match, generate syndrome b i t s from t h e r e s u l t of the E C C
compare.
Decode syndrome b i t s t o determine i f a s i n g l e - o r m u l t i p l e - b i t f a i l u r e
occurred.
I f a single-bit f a i l u r e occurred, c o r r e c t by i n v e r t i n g t h e f a i l i n g b i t i n
the d a t a word. Send the c o r r e c t e d word t o t h e r e q u e s t i n g u n i t .
If a multiple-bit o r o t h e r uncorrectable e r r o r occurred, send the
uncorrectable e r r o r response code t o t h e C P o r
the IOU.
A PP i n the
IOU
may then analyze the syndrome b i t s using t h e maintenance channel.