GIET-VM / Context Switch Handler

The ctx_handler.c​ and ctx_handler.h​ files define the data structure and functions used to handle context switching. They are prefixed by "_" to remind that they can only be executed by a processor in kernel mode.

A task context is an array of 64 words = 256 bytes. It contains copies of processor registers (when the task is preempted):

• GPR[i] : generally stored in slot (i). $0, $26 & $27 are not saved.
• HI & LO registers : used by multiply/divide instructions.
• CP0 registers : EPC, SR, CR, BVAR
• CP2 registers : PTPR

It contains some general informations associated to the task:

• TTY : TTY channel global index
• NIC : NIC channel global index
• CMA : CMA channel global index
• HBA : HBA channel global index
• DMA : DMA channel local index
• TIM : TIM channel local index
• PTAB : page table virtual base address
• LTID : Task local index (in scheduler)
• VSID : Virtual space index
• RUN : Task state (0 => sleeping / 1 => runnable )
• TRDID : Thread ID index (in vspace)

[0] *	[8] $8	[16] $16	[24] $24	[32] EPC	[40] TTY	[48] TRDID
[1] $1	[9] $9	[17] $17	[25] $25	[33] CR	[41] DMA	[49] *
[2] $2	[10] $10	[18] $18	[26] LO	[34] SR	[42] NIC	[50] *
[3] $3	[11] $11	[19] $19	[27] HI	[35] BVAR	[43] TIM	[51] *
[4] $4	[12] $12	[20] $20	[28] $28	[36] PTAB	[44] HBA	[52] *
[5] $5	[13] $13	[21] $21	[29] SP	[37] LTID	[45] CMA	[53] *
[6] $6	[14] $14	[22] $22	[30] $30	[38] VSID	[46] GTID	[54] *
[7] $7	[15] $15	[23] $23	[31] RA	[39] PTPR	[47] RUN	[55] *

void _ctx_switch()

This function performs a context switch between the running (calling) task and another runable task, using a round-robin sheduling policy between all tasks allocated to a given processor (static allocation). It selects the next runable task to resume execution. If the only runable task is the current task, return without context switch. If there is no runable task, the scheduler switch to the default "idle" task. The return address contained in $31 is saved in the current task context (in the ctx[31] slot), and the function actually returns to the address contained in the ctx[31] slot of the next task context.

void _ctx_eret()

The address of this function is used to initialise the return address in the "idle" task context.

void _idle_task()

This function is executed task when no other task can be executed.