Exec maintains task context and state information in a task-control data structure. Like most Exec structures, Task structures are dynamically linked onto various task queues through the use of an embedded Exec list Node structure (see the "Exec Lists and Queues" chapter). Any task can find its own task structure by calling FindTask(NULL). The C-language form of this structure is defined in the <exec/tasks.h> include file: struct Task { struct Node tc_Node; UBYTE tc_Flags; UBYTE tc_State; BYTE tc_IDNestCnt; /* intr disabled nesting */ BYTE tc_TDNestCnt; /* task disabled nesting */ ULONG tc_SigAlloc; /* sigs allocated */ ULONG tc_SigWait; /* sigs we are waiting for */ ULONG tc_SigRecvd; /* sigs we have received */ ULONG tc_SigExcept; /* sigs we will take excepts for */ UWORD tc_TrapAlloc; /* traps allocated */ UWORD tc_TrapAble; /* traps enabled */ APTR tc_ExceptData; /* points to except data */ APTR tc_ExceptCode; /* points to except code */ APTR tc_TrapData; /* points to trap code */ APTR tc_TrapCode; /* points to trap data */ APTR tc_SPReg; /* stack pointer */ APTR tc_SPLower; /* stack lower bound */ APTR tc_SPUpper; /* stack upper bound + 2*/ VOID (*tc_Switch)(); /* task losing CPU */ VOID (*tc_Launch)(); /* task getting CPU */ struct List tc_MemEntry; /* allocated memory */ APTR tc_UserData; /* per task data */ }; A similar assembly code structure is available in the <exec/tasks.i> include file. Most of these fields are not relevant for simple tasks; they are used by Exec for state and administrative purposes. A few fields, however, are provided for the advanced programs that support higher level environments (as in the case of processes) or require precise control (as in devices). The following sections explain these fields in more detail.
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