Environmental Calls

To use an environmental call, load the ID into register a0, and load any arguments into a1 - a7. Any return values will be stored in argument registers.

The following environmental calls are currently supported.

ID (a0)	Name	Description
1	print_int	prints integer in a1
4	print_string	prints the null-terminated string whose address is in a1
9	sbrk	allocates a1 bytes on the heap, returns pointer to start in a0
10	exit	ends the program
11	print_character	prints ASCII character in a1
13	openFile	Opens the file in the VFS where a pointer to the path is in a1 and the permission bits are in a2. Returns to a0 an integer representing the file descriptor.
14	readFile	Takes in: a1 = FileDescriptor, a2 = Where to store the data (an array), a3 = the amount you want to read from the file. Returns a0 = Number of items which were read and put to the given array. If it is less than a3 it is either an error or EOF. You will have to use another ecall to determine what was the cause.
15	writeFile	Takes in: a1 = FileDescriptor, a2 = Buffer to read data from, a3 = amount of the buffer you want to read, a4 = Size of each item. Returns a0 = Number of items written. If it is less than a3 it is either an error or EOF. You will have to use another ecall to determine what was the cause.
16	closeFile	Closes a file WIP
17	exit2	ends the program with return code in a1
18	fflush
19	feof
20	ferror
34	printHex	prints hex in a1

The environmental calls are intended to be somewhat backwards compatible with SPIM's syscalls.

As an example, the following code prints the integer 42 to the console:

addi a0 x0 1        # print_int ecall
addi a1 x0 42       # integer 42
ecall