ST20-C1 datasheet(23/205 Pages) STMICROELECTRONICS

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3 Architecture

chain. In addition, they allow for saturated arithmetic to be implemented rela-

tively easily.

A (non-sticky) carry bit is provided to allow efficient implementation of long

addition and subtraction. The carry bit is only manipulated by the

addc and

subc instructions allowing the other add instructions to be used in address for-

mation of multi-word values where carry propagation is required so that the

carry is not lost in the address formation evaluations.

• The user_mode bit indicates when the machine is handling a user process,

i.e. a process which is not an exception handler. The interrupt_mode bit indi-

cates when the machine is handling an interrupt, or a trap from an interrupt

handler and the trap_mode bit indicates when the machine is executing a trap

handler. An operating system may need to distinguish between modes to allow

it to perform scheduling activities from a trap handler. These bits are also

required to enable the

eret instruction to determine whether a signal to the

interrupt controller is required.

• The sleep bit indicates that the CPU is due to go to sleep, i.e. to turn off its

clocks and go into low power mode. This bit is set when the CPU detects there

is no user process to execute and is cleared when the CPU goes to sleep.

• The start_next_task bit when set causes the processor to attempt to run the

next process from the scheduling queue.

• The timeslice_enable bit and timeslice_count field are used for timeslicing,

as described in section 7.4.

The instructions to use the status register are described in section 4.12.

3.3.3

The work space pointer

All programs need somewhere to store local working data, e.g. local variables in the

application code. In the ST20 architecture, this local storage is termed the

work space

of the program.

The Wptr register is the local work space pointer, which holds the address of the stack

of the executing process. The stack is downward pointing, so space is allocated by

moving the Wptr to a lower address. This address is word aligned and therefore has

the two least significant bits set to z ero. When a process is descheduled, the Wptr is

stored as part of the process descriptor block, which is pointed to by Tdesc.

The Wptr is used as a base for addressing

local variables. A word offset from the

Wptr is the operand for the instructions

ldl (load local), stl (store local) and ldlp (load

local pointer).

The ST20-C1 simplifies the nor mal stack scheme by decoupling the load/store action

from the pointer update:

• Load-local and store-local instructions access values in the work space with

addresses relative to the Wptr, but do not change the value of Wptr.

• Separate instructions (ajw, gajw) are provided to update the work space

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