Practical Session 7

Co-RoutinesCo-Routines

• co-routines are assembly implementation of threads

• each co-routine decides to which co-routine to pass a control - unlike “usual” threads

Co-routine stateCo-routine state• co-routine is denoted by Coi, where i is ID of the co-

routine• scheduler is also a co-routine • co-routine suspends itself after some “time slice”• co-routine resumes a scheduler

• co-routine should save its current state before it suspends itself, in order to continue its execution later

• a state of co-routine includes:• stack state• registers• flags• stack pointer (ESP)• instructions pointer (EIP)

Co-routine structureCo-routine structure

STKSIZE equ 16*1024 ;(16 Kb)STK1: resb STKSIZE

CO1: dd Function1 ; pointer to co-routine codeFlags: dd 0 ; 0 if co-routine is not initialized, 1 otherwiseSP1: dd STK1 + STKSIZE ; pointer to co-routine stack

Co-routine structureCo-routine structure

Variable DeclarationVariable DeclarationSTKSZ equ 16*1024CODEP equ 0FLAGSP equ 4SPP equ 8

section .rodataalign 16global numco

numco: dd 3CORS: dd CO1

dd CO2dd CO3

section .dataalign 16

CO1: dd Function1 ; Structure for first co-routineFlags1: dd 0SP1: dd STK1+STKSZCO2: dd Function1 ; Structure for second co-routineFlags2: dd 0SP2: dd STK2+STKSZCO3: dd Function2 ; Structure for third co-routineFlags3: dd 0SP3: dd STK3+STKSZ

section .bssalign 16

CURR: resd 1SPT: resd 1SPMAIN: resd 1STK1: resb STKSZSTK2: resb STKSZSTK3: resb STKSZ

Co-routine initializationCo-routine initialization

Co-routine initialization:Co-routine initialization:save initial co-routine statesave initial co-routine state

‘bts’ instruction tests one bit of its first operand, whose index is given by the second operand, and stores the value of that bit in the carry flag (CF). In addition, ‘bts’ sets the bit to be 1.

Co-routine initializationCo-routine initializationinit_co_from_c : initialize a co-routine, which number is given as an argument from Csection .text

align 16extern printfglobal init_co_from_cglobal start_co_from_cglobal end_co

init_co_from_c:push EBPmov EBP, ESPpush EBXmov EBX, [EBP+8] ; EBX contains a number of the co-routine to be initializedmov EBX, [EBX*4+CORS] ; EBX contains a pointer to co-routine structure to be initializedcall co_initpop EBXpop EBPret

co_init:pushabts dword [EBX+FLAGSP],0 ; test if already initializedjc init_donemov EAX,[EBX+CODEP] ; get initial PCmov [SPT], ESP ; save original SPmov ESP,[EBX+SPP] ; get initial SPmov EBP, ESP ; also use as EBPpush EAX ; push initial "return" address (initial PC)pushf ; push flagspusha ; push all other registersmov [EBX+SPP],ESP ; save new SP in structuremov ESP, [SPT] ; restore original SP

init_done:poparet

Co-routine code (function)Co-routine code (function)

Scheduler co-routine functionScheduler co-routine functionScheduler function is started by main.

scheduler_Function:

pick up some thread ID imov EBX, [CORS + i*4] ; resumes Coicall resume

pick up some other thread ID jmov EBX, [CORS + j*4] ; resumes Cojcall resume…jmp end_co ; resume main

Co-routine resumeCo-routine resume• What should resume do?

• save a state of the current co-routine• resume a state of the next co-routine (EBX should contain a

pointer to it)

• Let’s look closer at the resume function:• after ‘call resume’ , returned address (i.e. EIP) is pushed • automatically into (co-routine) stack• we just have to save EFLAGS, ESP, and registers

resume:pushfpushamov EDX, [CURR]mov [EDX+SPP], ESP

Co-routine resumeCo-routine resume

Co-routine resumeCo-routine resume

EBX is pointer to co-init structure of co-routine to be resumedCURR holds a pointer to co-init structure of the current co-routine

resume:pushf ; save state of callerpushamov EDX, [CURR]mov [EDX+SPP],ESP ; save current SP

do_resume:mov ESP, [EBX+SPP] ; load SP for resumed co-routinemov [CURR], EBXpopa ; restore resumed co-routine statepopfret ; "return" to resumed co-routine!

Function1Function1

This function used as code for co-routines 0 and 1FMT1: db "Function1, co %lx, called by %lx, pass %ld", 10, 0

Function1:push dword 1push dword [CORS+8]push dword [CURR]push dword FMT1call printfadd ESP, 20mov EBX, [CORS+8] ; resume a schedulercall dword resume

push 2push dword [CORS+8]push dword [CURR]push dword FMT1call printfadd ESP, 20mov EBX, [CORS+8] ; resume a schedulercall dword resume

Function2Function2This function used as code for co-routine 2FMT2: db "Function2, co %lx, called by %lx, pass %ld", 10, 0

Function2:push dword 1push dword [CORS]push dword [CURR]push dword FMT2call printfadd ESP, 16mov EBX, [CORS] ; resume CO1call dword resume

push dword 2push dword [CORS+4]push dword [CURR]push dword FMT2call printfadd ESP, 20

mov EBX, [CORS+4] ; resume CO2call dword resumepush dword 3push dword [CORS]push dword [CURR]push dword FMT2call printfadd ESP, 16mov EBX, [CORS] ; resume CO1call dword resumepush dword 4push dword [CORS+4]push dword [CURR]push dword FMT2call printfadd ESP, 16mov EBX, [CORS+4] ; resume CO2call dword resumejmp end_co ; resume main

Start co-routine schedulerStart co-routine scheduler

• We start scheduling by suspending main function and starting (resuming) a first co-routine (scheduler)

• In our implementation, we call (from c) to start_co_from_c(2)

start_co_from_c:pushamov [SPMAIN], ESP ; save stack pointer of the main codemov EBX, [EBP+8] ; gets ID number of a scheduler structuremov EBX, [EBX*4 + CORS] ; gets a pointer to a scheduler structurejmp do_resume ; resume a scheduler co-routine

co-routine start and endco-routine start and endC-callable start of the first co-routine

start_co_from_c:push EBPmov EBP, ESPpushamov [SPMAIN], ESP ; save SP of main codemov EBX, [EBP+8] ; get number of co-routinemov EBX, [EBX*4+CORS] ; and pointer to co-routine structurejmp do_resume

End co-routine mechanism, back to c main

end_co:mov ESP, [SPMAIN] ; restore state of main codepopapop EBPret

Main.c#include <stdio.h>extern long numco;extern void end_co();extern void start_co_from_c(int num);extern void init_co_from_c(int num);extern void resume_from_c(int num);

main() {long i;for(i = 0; i < numco; i++)init_co_from_c(i); // initialize co-routinesprintf("After init\n");start_co_from_c(2); // start a scheduler co-routineprintf("All co-routines done\n");

}

Run example – data declarationRun example – data declaration

CURR

SPT

SPMAIN

STK1

STK2

STK3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

0 Flags1

SP1

Function1 CO2

0 Flags2

SP2

Function2 CO3

0 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .data

AfterAfter co-routine initializationco-routine initialization

CURR

SPT

SPMAIN

STK1

Registers

Flags

Function1

STK2

Registers

Flags

Function1

STK3

Registers

Flags

Function2

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .data

Resuming - right beforeResuming - right before

CO2 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

STK2

ESP ……….

……….

STK3

Registers

Flags

Addr3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

resume:pushf

pushamov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP, [EBX+SPP] mov [CURR], EBXpopapopfret

.bss .data

Resuming – resume is calledResuming – resume is called

CO2 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

STK2

ESP Addr2

……….

……….

STK3

Registers

Flags

Addr3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .dataresume:

pushfpusha

mov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP, [EBX+SPP] mov [CURR], EBXpopapopfret

Resuming – backup registersResuming – backup registers

CO2 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

ESP Registers STK2

Flags

Addr2

……….

……….

STK3

Registers

Flags

Addr3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .dataresume:

pushfpusha

mov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP, [EBX+SPP] mov [CURR], EBXpopapopfret

Resuming – backup stack pointer Resuming – backup stack pointer

CO2 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

ESP Registers STK2

Flags

Addr2

……….

……….

STK3

Registers

Flags

Addr3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .dataresume:

pushfpusha

mov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP, [EBX+SPP] mov [CURR], EBXpopapopfret

Resuming Resuming - - load stack pointer of resume co-routineload stack pointer of resume co-routine

CO2 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

Registers STK2

Flags

Addr2

……….

……….

STK3

ESP Registers

Flags

Addr3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .dataresume:

pushfpusha

mov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP,

[EBX+SPP] mov [CURR], EBXpopapopfret

Resuming – Resuming – set current co-routine variableset current co-routine variable

CO3 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

Registers STK2

Flags

Addr2

……….

……….

STK3

ESP Registers

Flags

Addr3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .dataresume:

pushfpusha

mov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP, [EBX+SPP] mov [CURR], EBXpopapopfret

Resuming – Resuming – restore a state of loaded co-routinerestore a state of loaded co-routine

CO3 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

Registers STK2

Flags

Addr2

……….

……….

STK3

ESP Addr3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .dataresume:

pushfpusha

mov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP, [EBX+SPP] mov [CURR], EBXpopapopfret

Resuming – Resuming – go to execute the loaded co-routinego to execute the loaded co-routine

CO3 CURR

SPT

SPMAIN

STK1

Registers

Flags

Addr1

Registers STK2

Flags

Addr2

……….

……….

STK3

3 numco

CO1 CORS

CO2

CO3

Function1 CO1

1 Flags1

SP1

Function1 CO2

1 Flags2

SP2

Function2 CO3

1 Flags3

SP3

0 COUNTER

3 MAX_ITER

.bss .dataresume:

pushfpusha

mov EDX, [CURR]mov [EDX+SPP],ESP

do_resume:mov ESP, [EBX+SPP] mov [CURR], EBXpopapopfret