Section A.2 OS Calls
Calling the operating system is done by the
ecall instruction. The operation to be carried out is specified by the a0 register. Potential arguments to the system call are passed in the other argument registers.| Action | a0 |
Description |
| print integer | 1 |
a1 = number to be printed. |
| print string | 4 |
a1 = address of ASCIIZ string. |
| convert integer | 5 | Return integer interpretation of null-terminated string from a1 in a0. |
| read integer | 6 | Return read number in a0. |
| read string | 8 | a1 = start address of memory area where characters are put. a2 = maximum number of characters to read. |
| exit | 10 | End program execution. |
| print character | 11 |
a1 = ASCII code of character to print. |
| read character | 12 | Return ASCII code of read character in a0. |
| enable terminal | 0x130 | Enables the VSCode terminal for receiving input. |
| read character poll | 0x131 |
a0=0 if all input read, a0=1 if waiting for input, a0=2 if a character was read, a1 contains the character |
There are more ecalls for specialized tasks:
| Action | a0 |
Description |
| heap allocation (sbrk) | 9 | Allocates a a1 large segment on the heap returned in a0
|
| open file | 13 |
a1 filename, a2 permissions, returns filehandle in a0
|
| read file | 14 | read a3 bytes from filehandle from a1, writes to address at a2, a0 contains number of bytes read |
| write file | 15 |
a1 filehandle, a2 buffer with data, a3 amount to write, a4 size of each item, returns a0 number of items written |
| close file | 16 |
a1 filehandle, return 0 if successfull, EOF otherwise |
| exit with error code | 17 |
a1 error code |
| flush | 18 |
a1 filehandle, return 0 if successfull, EOF otherwise |
| end of file | 19 |
a1 filehandle, return a0 is non-zero if file at end |
| error check | 20 |
a1 filehandle, return a0 is non-zero if file encountered errors |
| print hex | 34 |
a1 number to print in hex |
