TC1796
System Units (Vol. 1 of 2)
Introduction
User’s Manual 1-3 V2.0, 2007-07
Intro, V2.0
– Word = 32-bit quantity
– Double-word = 64-bit quantity
1.1.3 Reserved, Undefined, and Unimplemented Terminology
In tables where register bit fields are defined, the following conventions are used to
indicate undefined and unimplemented functions. Furthermore, types of bits and bit
fields are defined using the abbreviations as shown in Table 1-1.
Table 1-1 Bit Function Terminology
Function of Bits Description
Unimplemented,
Reserved
Register bit fields named 0 indicate unimplemented functions
with the following behavior.
• Reading these bit fields returns 0.
• Writing these bit fields has no effect.
These bit fields are reserved. When writing, software should
always set such bit fields to 0 in order to preserve compatibility
with future products.
Undefined,
Reserved
Certain bit combinations in a bit field can be marked “Reserved”,
indicating that the behavior of the TC1796 is undefined for that
combination of bits. Setting the register to such undefined bit or
bit field combinations may lead to unpredictable results. Such bit
combinations are reserved. When writing, software must always
set such bit fields to legal values as defined for it.
rw The bit or bit field can be read and written.
rwh As rw, but bit or bit field can be also set or cleared by hardware.
r The bit or bit field can only be read (read-only).
w The bit or bit field can only be written (write-only).
rh The bit or bit field can only be read. It can be also set or cleared
by hardware (typical example: status flags).
s Bits with this attribute are “sticky” in one direction. If their reset
value is once overwritten by software, they can be switched
again into there reset state only by a reset operation. Software
cannot switch this type of bit into its reset state by writing the
register. This attribute can be combined to ‘rws’ or ‘rwhs’.
f Bits with this attribute are readable only when they are accessed
by an instruction fetch. Normal data read operations will return
other values.
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