Maxim Integrated MAXQ622 - Manual (Page 20)

MAXQ612/MAXQ622 User’s Guide
2-16 Maxim Integrated
2.6 Memory Protection
The MAXQ612/MAXQ622 support privilege levels for code . When enabled, code memory is separated into three areas .
Each area has an associated privilege level . RAM/utility ROM are assigned privilege levels as well:
• Code in the system area can be confidential. Code in the user areas can be prevented from reading and writing
system code .
• The user loader can be protected from user application code.
The PRIV register reflects the current execution privilege . Hardware guarantees that the contents of PRIV are never
higher than the maximum privilege level of the memory area the code is running from . For example, if user code were
trying to set PRIV to high, this would be prevented by hardware . However, any code can decide to lower the privilege
level at any time (see Equation 1) .
PRIV = min(maxprivilege(IP), PRIV) (Equation 1)
The bit contents of the PRIV register are shown in Table 2-5 . The convenient constants high/medium/low are defined
in Table 2-6, but all values from 00b to 11b can be used .
In addition to the PRIV register, the privilege level can also be set by writing to PRIVT0 and PRIVT1 in sequence . Again,
hardware guarantees that the contents of PRIVT0 are never higher than the maximum privilege level of the memory
area the code is running from .
When writing to PRIVT1, hardware modifies the PRIV register based on Equation 2 .
PRIV = min(PRIVT0, argument, maxprivilege(IP)) (Equation 2)
This means that, when using PRIVT[1:0], the privilege level cannot be raised unless all code between the writes to
PRIVT0 and PRIVT1 executes . Writing to PRIV automatically resets PRIVT0 to low .
In addition to the PRIV register, the privilege level can also be set by writing to PRIVT0 and PRIVT1 in sequence . Again,
hardware guarantees that the contents of PRIVT0 are never higher than the maximum privilege level of the memory
area the code is running from .
When writing to PRIVT1, hardware modifies the PRIV register based on
PRIV’ = min(PRIVT0, argument, maxprivilege(IP))
Table 2-4. Memory Areas and Associated Maximum Privilege Levels
Table 2-5. PRIV Register Bit Definitions
Table 2-6. Privilege Level Constants
AREA PAGE ADDRESS MAXIMUM PRIVILEGE LEVEL
System 0 to ULDR-1 High
User Loader ULDR to UAPP-1 Medium
User Application UAPP to top Low
Utility ROM N/A High
Other (RAM) N/A Low
BIT 3 2 1 0
MEANING System Write System Read User Loader Write User Loader Read
BIT 3 2 1 0
HIGH 1 1 1 1
MEDIUM 0 0 1 1
LOW 0 0 0 0