qemu-system-riscv64 -machine virt -bios build/bin/kernel -serial mon:stdio

qemu-system-riscv64 -machine virt -cpu rv64 -smp 4 -m 128M -drive if=none,format=raw,file=hdd.dsk,id=foo -device virtio-blk-device,scsi=off,drive=foo -nographic -serial mon:stdio -bios none -device virtio-rng-device -device virtio-gpu-device -device virtio-net-device -device virtio-tablet-device -device virtio-keyboard-device -kernel build/bin/kernel

qemu-system-riscv64 -machine virt -cpu rv64 -smp 1 -m 128M -device virtio-keyboard-device -bios none -kernel build/bin/kernel.elf -serial mon:stdio

qemu-system-riscv64 -machine virt -cpu rv64ima -smp 1 -m 128M
-device virtio-keyboard-device
-bios none -kernel build/bin/kernel.elf -serial mon:stdio -gdb tcp::1234 -S

/opt/riscv-ima/bin/riscv64-unknown-elf-gdb build/bin/kernel.elf -ex "target remote :1234" /opt/riscv-ima/bin/riscv64-unknown-elf-objdump -d build/bin/kernel.elf

Resources

• https://wiki.osdev.org/RISC-V_Bare_Bones
• https://osblog.stephenmarz.com/

RISC-V

Summary of the data provided here (https://riscv.org/technical/specifications/).

Privilege levels

Encoding	Name	Abbr
00	User	U
01	Supervisor	S
10	Reserved
11	Machine	M

CSRs

• satp
  • supervisor address translation and protection
  • contains root page table and other VM info
• mepc
  • machine expection pc
  • stores address of instruction on exception
  • mret uses this value to jump back to executing code
• mtval
  • exception-specific information to assist software in handling the trap
• mtvec
  • address of trap handler, must be 4 byte aligned
  • lowest 2 bits specify mode, 0 is direct, 1 is vectored
• mcause
• mstatus
  • machine status
  • MPP(11) - privilege mode
  • MPIE(7) - machine pending interrupt enable
    • must match MIE
  • MIE(3) - machine interrupt enable
    • must match MPIE
• mhartid
  • ID of hardware threat (hart)
• mie
  • machine interrupt enable
    • MSIE(3) - enable machine software interrupts
    • MTIE(7) - enable machine timer interrupts
    • MEIE(11) - enable machine external interrupts
• mscratch
  • machine scratch register

Special instructions

• mret
  • jumps to mepc
  • enables interrupts for the current privilege
  • sets the current privileged level to MPP
  • sets MPP to U.
• csrw csr, reg
  • write CSR
• csrr reg, csr
  • read CSR
• csrrw dst, csr, src
  • place src in csr, read csr into dst

Traps

• MSB of mcause CSR determeines if interrupt or exception
• The rest of mcause is the exception code

Interrupts

Exception Code	Description
0	Reserved
1	Supervisor software interrupt
2	Reserved
3	Machine software interrupt
4	Reserved
5	Supervisor timer interrupt
6	Reserved
7	Machine timer interrupt
8	Reserved
9	Supervisor external interrupt
10	Reserved
11	Machine external interrupt
12-15	Reserved
>=16	Designated for platform use

Exceptions

Exception Code	Description
0	Instruction address misaligned
1	Instruction access fault
2	Illegal instruction
3	Breakpoint
4	Load address misaligned
5	Load access fault
6	Store/AMO address misaligned
7	Store/AMO access fault
8	Environment call from U-mode
9	Environment call from S-mode
10	Reserved
11	Environment call from M-mode
12	Instruction page fault
13	Load page fault
14	Reserved
15	Store/AMO page fault
16-23	Reserved
24-31	Designated for custom use
32-47	Reserved
48-63	Designated for custom use
>=64	Reserved

VIRT machine in QEMU

Memory Map

Region	Base Address	Size
DEBUG	0x0	0x100
MROM	0x1000	0x1_1000
TEST	0x10_0000	0x1000
CLINT	0x200_0000	0x1_0000
PLIC	0xc00_0000	0x400_0000
UART0	0x1000_0000	0x100
VIRTIO	0x1000_1000	0x1000
DRAM	0x8000_0000	0x0
PCIE_MMIO	0x4000_0000	0x4000_0000
PCIE_PIO	0x0300_0000	0x0001_0000
PCIE_ECAM	0x3000_0000	0x1000_0000

memory mapped registers

Register	address	use
mtime	0x0200bff8	number of cycles
mtimecmp	0x02004000	machine timer interrupt is fired when this is greater than or equal to mtime

Clock

Clock in QEMU is 10 MHz or 10,000,000 cycles a second.

UART - NS16550a