Print a string
The first program that talks to anybody: a service number in $v0, an argument in $a0, and the one instruction that hands the request over.
The first program of the ladder that anybody outside the editor could see the result of. It prints a line, then prints a second line with a number at the end of it, and ends itself. The answer is in the console panel under the program instead of in a register.
Everything up to here left its result in the registers or in memory, because printing is a request to the environment and MIPS has one instruction for making requests. This is that instruction.
You need to know: the "syscall" lecture. What is new here is the whole shape of a request, the
service number in $v0 says what you want, $a0 carries the argument, and syscall hands it over.
syscall takes no operands at all. Everything about the request is in the registers before it runs,
which is why every one of these blocks is a li $v0, then whatever the service reads, then the
instruction. The M68K writes the same request as a task number in d0.b and a trap #15; the shape
is the same and only the register names and the numbers differ.
.asciiz "Hello, world!\n" writes fifteen bytes: thirteen characters, the newline that \n stands
for, and the zero the z in the directive adds. Service 4 walks the string from $a0 until it
reads that zero, so .ascii without it would print whatever came next in the data section as well.
Three services print here and each takes one thing. Service 4 prints a string, service 1 prints a
signed 32 bit number, and service 11 prints one character. There is nothing that prints a string and
a number together, which the M68K has as task 17, so the line The answer is 42 is written by two
requests and the newline after it by a third.
li $v0, 10 and syscall is service 10, exit, and it is what stops the program. Take those two
lines out and execution carries on into whatever instructions follow them in the text section, which
for a program with its subroutines written under main means running the subroutines.
Try changing greeting: .asciiz "Hello, world!\n" to .asciiz "Hello, world!". The console reads
Hello, world!The answer is 42 all on one line, because nothing prints a newline for you: service 4
prints exactly the bytes you gave it.