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env: fix strlen and add more comments

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magical 2023-04-23 15:46:34 -07:00
parent 84ce44330c
commit b267b3f0b2
1 changed files with 10 additions and 21 deletions
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31
env.s
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@ -21,19 +21,22 @@ _start:
addi s1, sp, 16 # add 8 for argc, 8 for the NULL at the end of argv addi s1, sp, 16 # add 8 for argc, 8 for the NULL at the end of argv
# compute start of envp # compute start of envp
# we could use a sh3add instruction # we could use a sh3add instruction (from the bitmanip extension)
# but the tools i'm using don't support the bitmanip extension # but it's not supported by the tools i'm using
#sh3add s1, sp, s0 #sh3add s1, sp, s0
sll t0, s0, 3 # argc*8 sll t0, s0, 3 # argc*8
add s1, s1, t0 add s1, s1, t0
# allocate space for an iovec # allocate space for two iovecs on the stack
# an iovec is just a pointer and a length
addi sp, sp, -(8+8)*2 addi sp, sp, -(8+8)*2
# initialize the second iovec with "\n", 1
la t0, newline la t0, newline
sd t0, 16(sp) sd t0, 16(sp)
li t1, 1 li t1, 1
sd t1, 24(sp) sd t1, 24(sp)
loop: loop:
# check for null pointer, which signals the end of envp # check for null pointer, which signals the end of envp
ld a1, 0(s1) ld a1, 0(s1)
@ -46,28 +49,14 @@ loop:
lbu t0, 0(t1) lbu t0, 0(t1)
addi t1, t1, 1 addi t1, t1, 1
bnez t0, strlen bnez t0, strlen
addi t1, t1, -1 # NUL
sub a2, t1, a1 sub a2, t1, a1
sd a2, 8(sp) sd a2, 8(sp)
/*
# print string
li a7, 64
li a0, 1
# a1 already set to start of string
# a2 already set to string length
ecall
# print newline
li a7, 64
li a0, 1
la a1, newline
li a2, 1
ecall
*/
li a7, 66 # sys_writev li a7, 66 # sys_writev
li a0, 1 li a0, 1 # stdout
mv a1, sp mv a1, sp # pointer to iovec
li a2, 2 li a2, 2 # number of iovecs
ecall ecall
bltz a0, write_error bltz a0, write_error