Syscalls

In RISC-V, syscalls allow a program to request services from the execution environment.

This is done using the ecall instruction.

In the ÆRIS simulator, syscalls are mainly used for console input and output.

Unlike a real operating system, ÆRIS does not have a kernel mode, so ecall is handled directly by the simulator.

---

How ecall Works

A syscall follows three steps:

1. Set the syscall code in register a7
2. Pass the arguments in registers (a0, a1, …)
3. Execute the ecall instruction

Example:

li a7, 1
li a0, 42
ecall

This prints the number 42.

---

Available Syscalls

Code	Name	Description
1	PrintInt	Prints the integer in a0
2	PrintString	Prints a string from the address in a0
3	ReadInt	Reads an integer from the console
4	ReadString	Reads a string from the console

---

PrintInt (1)

Prints the integer value stored in a0.

Example:

li a7, 1
li a0, 123
ecall

Output:

123

---

PrintString (2)

Prints a sequence of characters from the address in a0.

In ÆRIS, strings do not have an automatic null terminator.

Example:

.data
msg: .ascii "Hello"

.text
main:
    li a7, 2
    la a0, msg
    ecall

---

ReadInt (3)

Reads an integer from the user.

Example:

li a7, 3
ecall

Behavior:

1. The simulator pauses execution
2. The user enters a number
3. The value is stored in a0

---

ReadString (4)

Reads a sequence of characters from the user and stores it in memory.

Arguments:

Register	Description
a0	Buffer address
a1	Maximum size

Example:

.data
buffer: .word 0,0,0,0,0,0,0,0

.text
main:
    li a7, 4
    la a0, buffer
    li a1, 32
    ecall

Behavior:

• The user types a string
• The characters are written in memory starting at a0
• At most a1 bytes are stored