Model-specific configuration registers
Placement of configuration values

"~" means the bit is a negated boolean. Sometimes values overlap,
depending on MCU model.

In STC89 series, there is only a single MCS byte.

MCS0
----

MSB     7       6       5       4       3       2       1       0       LSB
        ~WDEN   XRAM    ALE     OSCG    ~EERE   ~BSLD   0       ~CPU6T

~WDEN := watchdog enable after power-on-reset
XRAM := enable access to internal XRAM
ALE := enable ALE pin function (otherwise, it's just regular GPIO)
OSCG := high oscillator gain
~EREE := enable eeprom erase next time MCU is programmed
~BSLD := enable BSL pin detect; i.e. BSL is only enabled if P1.0/P1.1
         (or others, depends on MCU model) are held low on POR.
~CPU6T := enable double speed (6T cycles instead of 12T cycles) mode