A simple goto ring3 code for unicorn

boot32.S

# GDTR address
GDTR_ADDR = 0xc0001000
# user mode entry table
# - entry point
# - stack pointer
ENTRY_TABLE = 0xc0002000

# RING0_CS selector
KERNEL_CS_INDEX = 0x8

# RING3_DS selector
USER_DS_INDEX = 0x1b

.code32
.global kernel_start

kernel_start:
    lgdt GDTR_ADDR; 
    movl $0x174, %ecx;
    movl $KERNEL_CS_INDEX, %eax;
    xorl %edx, %edx;
    wrmsr;
    movl $ring3, %edx;
    # kernel sp will not be used
    movl $0x0, %ecx;
    sysexit;

# now we are in ring 3, setup proper segement register
# and we are ready to go
ring3:
    xorl %eax, %eax;
    mov %ax, %fs;
    mov $USER_DS_INDEX, %ax;
    mov %ax, %ds;
    mov %ax, %gs;
    mov %ax, %es;
    movl $ENTRY_TABLE, %eax;
    movl 0x4(%eax), %esp;
    jmp *(%eax);

link.ld

OUTPUT_FORMAT("binary")
INPUT(boot_32.o)
ENTRY(kernel_start)
OUTPUT(boot_32.bin)
SECTIONS
{
    .text 0xc0003000 :
    {
        *(.text)
    }
}

Makefile

CC := gcc
LD := ld
ASFLAGS := -c
LDFLAGS := -T link.ld

all: boot_32.bin

boot_32.bin: boot_32.o
	@${LD} ${LDFLAGS}
	@echo "LINK $@"

boot_32.o: boot_32.S
	@${CC} ${ASFLAGS} -o $@ $<
	@echo "AS $@"
	@objcopy -j .text $@
	@echo "STRIP $@"

clean:
	@rm -v *.o *.bin 2>/dev/null