[OS][MIT 6.828] Lab 3: User Environments(i.e., Processes)

Part A: User Environments and Exception Handling

1. User Environments

struct used to represent an environment

struct Env {
	struct Trapframe env_tf;	// Saved registers
	struct Env *env_link;		// Next free Env
	envid_t env_id;			// Unique environment identifier
	envid_t env_parent_id;		// env_id of this env's parent
	enum EnvType env_type;		// Indicates special system environments
	unsigned env_status;		// Status of the environment
	uint32_t env_runs;		// Number of times environment has run

	// Address space
	pde_t *env_pgdir;		// Kernel virtual address of page dir
};

three main global variables pertaining to environments:

struct Env *envs = NULL;		// All environments
struct Env *curenv = NULL;		// The current env
static struct Env *env_free_list;	// Free environment list

functions used for running environments

int envid2env(envid_t envid, struct Env **env_store, bool checkperm) // Converts an envid to an env pointer.
int env_alloc(struct Env **newenv_store, envid_t parent_id)  // alloc an Env from array envs, init variables in Env.
      static int env_setup_vm(struct Env *e) // Initialize the KERNEL virtual memory layout for environment e. ONLY used in env_alloc()
static void region_alloc(struct Env *e, void *va, size_t len) // Allocate len bytes of physical memory for environment e, and map it at virtual address va.
void env_create(uint8_t *binary, enum EnvType type) // create env ONLY during kernel initialization, later we should use fork
      static void load_icode(struct Env *e, uint8_t *binary) // only used in env_create()
void env_free(struct Env *e) // Frees env e and all memory it uses.
void env_destroy(struct Env *e)
void env_pop_tf(struct Trapframe *tf)
void env_run(struct Env *e)

summary of the creation of an environment

1. Allocate an Env from array envs. Implemented by env_alloc().
2. Initialize env_pgdir field of Env, actually, Initialize the virtual memory layout above UTOP including. Implemented by env_setup_vm().
3. Initialize the rest fields of Env, except env_tf.tf_eip. Implemented by env_alloc().
4. Load executable image into memory, initialize env_tf.tf_eip field. Implememted by load_icode().
5. Allocate 1 page memory for the initial stack and map it at USTACKTOP - PGSIZE. Implemented by region_alloc().

2. Exception Handling

summary of the function chain when handle a trap

1. t_*() :Defined by macros TRAPHANDLER and TRAPHANDLER_NOEC in trapentry.S.
2. _alltraps() :All fiels of trap frame are saved up to here.
3. trap()
4. trap_dispatch()
5. specific funtion corresponding to each trap

Part B: Page Faults, Breakpoints Exceptions, and System Calls

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