Use direct value for mode in fopen() instead of pointer -- mode is always 1 char so no need to store mode letters in memory

file.asm

@@ -18,7 +18,7 @@ section .text
 	global	fclose
 	global	fwrite
 
-;----- fopen(const char *filename, const char *mode) -----;
+;----- fopen(const char *filename, const char mode) -----;
 ; return value: pointer to file or value < 0 (neg errno) if file failed to open
 ;              ┌─────────────┬───────────────────────────────┐
 ;              │fopen() mode │ open() flags                  │
@@ -39,17 +39,17 @@ fopen:
 	push	rbp
 	mov	rbp, rsp
 
-	cmp	byte [rsi], 'r'
+	cmp	sil, 'r'
 	je	.setMode_r
-	cmp	byte [rsi], 'w'
+	cmp	sil, 'w'
 	je	.setMode_w
-	cmp	byte [rsi], 'a'
+	cmp	sil, 'a'
 	je	.setMode_a
-	cmp	byte [rsi], 'R'
+	cmp	sil, 'R'
 	je	.setMode_R
-	cmp	byte [rsi], 'W'
+	cmp	sil, 'W'
 	je	.setMode_W
-	cmp	byte [rsi], 'A'
+	cmp	sil, 'A'
 	je	.setMode_A
 	mov	rax, -22
 	jmp	.quit

tests.asm

@@ -149,7 +149,6 @@ section	.rodata
 	; fopen(), fwrite(), fclose()
 	testFile1	db	"testFile1.txt",EOS
 	fileStr1	db	"Hello file world!",EOS
-	fileMode_w	db	"w",EOS
 	msgFileB	db	NL,"# fopen(), fwrite(), fclose()",EOS
 	msgFileB1	db	"TEST fopen('testFile1.txt', 'w'): %d",NL,EOS
 	msgFileB2	db	"TEST fwrite(filePointer1, fileStr1): %d",NL,EOS
@@ -733,7 +732,7 @@ main:
 
 	; TEST: fopen()
 	lea	rdi, [rel testFile1]
-	lea	rsi, [rel fileMode_w]
+	mov	rsi, 'w'
 	call	fopen
 	mov	r15, rax ;store filepointer to R15
 	mov	rsi, rax