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- My Projects
examples- rainbow
- rainbow.s
- ansi
- ansi.s
- mandelbrot
- mandelbrot.s
- input
- input.s
- hello
- hello.s
- fibonacci
- fibonacci.s
- kernel
- rtc.s
- random.s
- virtio.s
- keyboard.s
- framebuffer.s
- const.s
- syscall.s
- trap.s
- memory.s
- console.s
- paging.s
- start.s
- kmain.s
- endian.s
- util.s
- stdlib.s
- irq.s
- fdt.s
- elf.s
- stack.s
Console output. When you assemble something and stub your toe slightly, those errors will appear here.
pc ra sp gp tp t0 t1 t2 t3 t4 t5 t6 s0 s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 s11 a0 a1 a2 a3 a4 a5 a6 a7 ft0 x ft1 x ft2 x ft3 x ft4 x ft5 x ft6 x ft7 x ft8 x ft9 x ft10 x ft11 x fs0 x fs1 x fs2 x fs3 x fs4 x fs5 x fs6 x fs7 x fs8 x fs9 x fs10 x fs11 x fa0 x fa1 x fa2 x fa3 x fa4 x fa5 x fa6 x fa7 x sscratch sepc scause stval
Address Code Disassembly Line Source 
Address Value (+0) Value (+4) Value (+8) Value (+12) Value (+16) Value (+20) Value (+24) Value (+28) Printed 
Name Address 
Environment Calls
Environment Calls are special higher-level useful actions that can be used in strained environments such as our bare-metal situation. They are often called System Calls. Many machines offer these functions as a means of aiding development without having to write everything from scratch. (Phew!! Of course, you can still see their implementations if you look at the
syscall.sfile in the kernel source code. Everything that runs in our little virtual machine is written in assembly and built the same way as your code!)To invoke these, which are implemented by the kernel, you follow the instructions provided in the table and description sections below for the environment call you wish to use. Then you use the special
ecallinstruction which will call into the kernel and perform the requested action before returning to your own code.For example, printing a number:
li a7, 4 # Select environment call 4 (print integer) li a0, 42 # Pass in arguments using the a1 register (see the table) ecall # Invoke the environment call (it will print '42')Other environment calls may have more or no arguments. Some may provide results in various registers, typically
a0. Refer to the table below for more information.Overview
a7 Name Sets (if any) Arguments Description 1 print integer a0: integer to printPrints the integer given in a1. It does not print a newline.2 print float fa0: 32-bit float to printPrints the 32-bit floating point value given in fa0. It does not print a newline.3 print double fa0: 64-bit double to printPrints the 64-bit floating point value given in fa0. It does not print a newline.4 print string a0: address of stringPrints the null-terminated string given in a0. It does not print a newline.5 read integer a0: integer read8 read string a0: characters reada0: address of buffera1: maximum number of characters to read.9 sbrk a0: number of bytes to allocate10 exit a0: exit codeTerminates the program and powers down the machine. 11 print character a0: character to printPrints the character given in a0.30 system time a0: milliseconds since bootGets the number of milliseconds since booting the machine. Will not be incredibly accurate. 34 print integer in hexadecimal a0: integer to printPrints the integer given in a1as an 8 or 16 digit hexadecimal number, padding with zeros if necessary. It does not print a newline.35 print integer in binary a0: integer to printPrints the integer given in a1as a 32 or 64 digit binary numbder, padding with zeros if necessary. It does not print a newline.40 set seed a0: id of generatorSets the given random number generator to the given seed. 41 random word a0: random worda0: The id of the generator to use.Descriptions and Usage
print integer
Set
a7to 1.Prints the given integer in
a0.li a7, 1 # Select environment call 1 (print integer) li a0, 42 # Pass in arguments using the a0 register (see the table) ecall # Invoke the environment call (it will print '42')print string
Set
a7to 4.Prints the given string whose address is within
a0.li a7, 4 # Select environment call 4 (print string) li a0, str # Pass in arguments using the a0 register (see the table) ecall # Invoke the environment call (it will print 'Hello!') .data str: .string "Hello!"read integer
Set
a7to 5.Waits for something to be typed in and parses that as an integer which it stores in
a0.li a7, 5 # Select environment call 5 (read integer) ecall # Invoke the environment call (it will wait here until something is typed in) # Now, a0 is the number typed in (or your program errors if the input was not a number!) # Let's double the number with a shift left sll a0, a0, 1 # While a0 is still the number we care about ... let's print it out li a7, 1 # Now, we will print it out using the print integer environment call ecall # Prints the number (double the input) back out! (Remember, a0 is still that number!)read string
Set
a7to 8.Waits for something to be typed in (ends with an 'enter' press) and writes it to the buffer given in
a0.li a7, 8 # Select environment call 8 (read string) la a0, buff # Give it the address of our memory we want to use to write the string to li a1, 99 # We can write up to 99 characters into our buffer ecall # Invoke the environment call (it will wait for a line to be entered) # Now, our buffer in `buff` is filled with the line just typed in # Let's print it out again li a7, 4 # Use the print string ecall la a0, str # Prints a helpful string ecall li a7, 4 # Use the print string ecall again la a0, buff # Prints the typed in string ecall .data buff: .fill 100, 1, 0 # Fills 100 bytes with 0s str: .string "\nYou typed in: "