Environmental Calls - kvakil/venus GitHub Wiki

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
17	exit2	ends the program with return code 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