Execution Units
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
26-57
26.4.5.9 AFEU Context
This section provides additional information about the AFEU context memory and its related
pointer register.
26.4.5.9.1 AFEU Context Memory
The S-Box memory consists of 256 bytes of SRAM, each readable and writable as part of a 64-bit
set. Do not write the S-Box contents with data unless that data is previously read from the S-Box.
Context data should only be written if the ‘prevent permutation’ mode bit in the AFEU Mode
Register is set (see Section 26.4.5.1, AFEU Mode Register, on page 26-54). After context data,
the context length must be written to the context/data length register (see Section 26.5.10.3,
AFEU Context/Data Size Register (AFEUCDSR), on page 26-152). After this, message data can
be written. If the Context Registers are written during message processing or the ‘prevent
permutation’ bit is not set, a context error will be generated.
Valid context data can only be read after Reading context data before the module is done will
generate an error interrupt.
Context data can be written and read either via the Context Registers or via the input and output
FIFOs. The user specifies which through the CS bit of the AFEU Mode Register (see
Section 26.4.5.1, AFEU Mode Register). See Section 26.4.5.10.1, AFEU FIFOs for more about
addressing the FIFOs.
26.4.5.9.2
AFEU Context Memory Pointer Register
The context memory pointer register holds the internal context pointers that are updated with
each byte of message processed. These pointers correspond to the values of I, J, and Sbox[I+1] in
the ARC-4 algorithm. If this register is written during message processing, a context error will be
generated.
When performing ARC-4 operations, the user has the option of performing a new S-Box
permutation per packet, or unloading the contents of the S-box (context) and reloading this
context prior to processing of the next packet. The S-Box contents (256 bytes) plus the three
bytes of the context memory pointers are unloaded and reloaded via the AFEU FIFOs.
AFEU Context consists of the contents of the S-Box, as well as three counter values, which
indicate the next values to be used from the S-Box. Context must be loaded in the same order in
which it was unloaded.
26.4.5.10 AFEU Key Registers
AFEU uses two write-only key registers to guide initial permutation of the AFEU S-Box, in
conjunction with the AFEU Key Size Register. AFEU performs permutation starting with the
first byte of key register 0, and uses as many bytes from the two key registers as necessary to
Summary of Contents for MSC8144E
Page 1: ...MSC8144E Reference Manual Quad Core Media Signal Processor MSC8144ERM Rev 3 July 2009 ...
Page 48: ...MSC8144E Reference Manual Rev 3 xlviii Freescale Semiconductor ...
Page 86: ...MSC8144E Reference Manual Rev 3 1 38 Freescale Semiconductor Overview ...
Page 168: ...MSC8144E Reference Manual Rev 3 3 60 Freescale Semiconductor External Signals ...
Page 242: ...MSC8144E Reference Manual Rev 3 5 26 Freescale Semiconductor Reset ...
Page 314: ...MSC8144E Reference Manual Rev 3 8 24 Freescale Semiconductor General Configuration Registers ...
Page 414: ...MSC8144E Reference Manual Rev 3 10 14 Freescale Semiconductor MSC8144E SC3400 DSP Subsystem ...
Page 452: ...MSC8144E Reference Manual Rev 3 11 38 Freescale Semiconductor Internal Memory Subsystem ...
Page 520: ...MSC8144E Reference Manual Rev 3 12 68 Freescale Semiconductor DDR SDRAM Memory Controller ...
Page 1070: ...MSC8144E Reference Manual Rev 3 21 28 Freescale Semiconductor Timers ...