IBM 990 - Manual (Page 55)

Chapter 2. System structure and design 43
Figure 2-13 Dual (asymmetric) processor design
Each PU has a dual processor and each processor has its own Instruction Unit (I-Unit) and
Execution Unit (E-Unit), which includes the floating point function. The instructions are
executed asymmetrically (not exactly in parallel) on each processor and compared after
processing.
This design simplifies error detection during instruction execution, saving additional circuits
and extra logic required to do this checking. The z990 servers also contain error-checking
circuits for data flow parity checking, address path parity checking, and L1 cache parity
checking.
Compression Unit on a chip
Each z990 PU has a Compression Unit on the chip, providing excellent hardware
compression performance. The Compression Unit is integrated with the CP Assist for
Cryptographic Function, benefiting from combining the use of buffers and interfaces.
CP Assist for Cryptographic Function
Each z990 PU has a CP Assist for Cryptographic Function on the chip. The assist provides
high performance hardware encryption and decryption support for clear key operations. To
that end, five new instructions are introduced with the cryptographic assist function.
The CP Assist for Cryptographic Function offers a set of symmetric cryptographic functions
that enhance the encryption and decryption performance of clear key operations for SSL,
VPN, and data storing applications that do not require FIPS 140-2 level 4 security. The
cryptographic architecture includes DES, T-DES data encryption and decryption, MAC
message authorization, and SHA-1 hashing.
The CP Assist for Cryptographic Function complements public key (RSA) functions and the
secure cryptographic operations provided by the PCIXCC cryptographic coprocessor card.
See Chapter 5, “Cryptography” on page 119 for more information about the cryptographic
features on the z.990.
Processing Unit (PU)
Dual processor
I-Unit
E-Unit
Floating Point function
Simple yet complete error detection
mechanism
Data flow - parity checked
Address paths - parity checked
L1 Cache - parity checked
Processor logic (I - E - F) -
Duplicated, then compared output.
Error detection for mis-compare
I-Unit
E-Unit
I-Unit
B-Unit
E-Unit
R-Unit
Floating p
Fixed p
To B-Unit To B-Unit
From E-Unit From E-Unit
COMPARE
To L2 Cache
From L2 Cache
Floating p
Fixed p
Error
Detection
L1
Cache