From cc214ed5e50ab0a6fdc37308bb932579d4a08990 Mon Sep 17 00:00:00 2001
From: Kwarde <kevin@kwarde.com>
Date: Sun, 6 Jul 2025 20:09:21 +0200
Subject: [PATCH] Adds basic printf() (currently only support '%%')

---
 src/console.asm | 118 +++++++++++++++++++++++++++++++++++++++++++++++-
 src/tests.asm   |  21 +++++++++
 2 files changed, 137 insertions(+), 2 deletions(-)

diff --git a/src/console.asm b/src/console.asm
index 438352d..b6c0aaa 100644
--- a/src/console.asm
+++ b/src/console.asm
@@ -3,10 +3,15 @@
 extern	strlen
 
 section .rodata
-	mNL	db	NL
+	mNL		db	NL
+	printfBuffLen	equ	128
+section .bss
+	printfBuff	resb	printfBuffLen
+	printfArgs	resq	5
 section .text
 	global	print
 	global	puts
+	global	printf
 
 ;----- print(*str[]) -----;
 ; Prints given string to the console to stdout
@@ -31,7 +36,10 @@ print:
 ; Prints given string to the console to stdout and prints a new line
 ; Return value: Amount of printed characters
 ; Used registers:
-;
+;	rax*	syscall >> (ret) amount of printed characters
+;	rdi*	(arg) Pointer to str[] >> syscall arg (fd)
+;	rsi*	syscall arg (pointer to str[])
+;	rdx*	syscall arg (length of str[]+1) 
 puts:
 	sub	rsp, SIZE_QWORD
 	call	print
@@ -45,3 +53,109 @@ puts:
 	inc	rax
 	add	rsp, SIZE_QWORD
 	ret
+
+;----- printf(*format[], ...) -----;
+; Formats and prints given string to console to stdout
+; Return value: Amount of printed characters
+; Supported specifiers:
+;	%%	Literal percentage sign
+; <!> Unsupported specifiers are printed as-is
+; Used registers:
+;	rax	(ret) amount of printed characters
+;	rdi	(arg) pointer to format[] to format and print
+;	rsi	(optional arg)
+;	rdx	(optional arg) >> Keeps track of amount of processed format specifiers
+;	rcx	(optional arg)
+;	r8	(optional arg) >> Used for moving characters
+;	r9	(optional arg) >> Keeps track of where to jump after flushing buffer
+;	r10*	Keeps track of current index of printfBuff
+;	r11*	Keeps track of total characters (return value)
+printf:
+	push	rbp
+	mov	rbp, rsp
+	sub	rsp, SIZE_QWORD
+
+	cmp	byte [rdi], EOS
+	je	.emptyStr
+	
+	xor	r10, r10
+	xor	r11, r11
+
+	; Store arguments to memory - easier to load data + more available registers (= less stack usage)
+	mov	[printfArgs + SIZE_QWORD * 0], rsi
+	mov	[printfArgs + SIZE_QWORD * 1], rdx
+	mov	[printfArgs + SIZE_QWORD * 2], rcx
+	mov	[printfArgs + SIZE_QWORD * 3], r8
+	mov	[printfArgs + SIZE_QWORD * 4], r9
+
+	.process:
+		cmp	byte [rdi], EOS
+		je	.wrapup
+		mov	r9b, 0
+		cmp	r10, printfBuffLen-1
+		je	.flushBuffer
+		.flushReturn_0:
+		cmp	byte [rdi], '%'
+		je	.argReplacement
+		mov	r8b, [rdi]
+		mov	[printfBuff+r10], r8b
+		inc	r10
+		inc	r11
+		inc	rdi
+		jmp	.process
+
+		.argReplacement:
+		cmp	byte [rdi + 1], EOS
+		je	.wrapup
+		cmp	byte [rdi + 1], '%'
+		je	.rep_pct
+
+		;--- Invalid ---;
+		mov	r9w, word [rdi]
+		mov	[printfBuff+r10], r9w
+		add	rdi, 2
+		add	r10, 2
+		add	r11, 2
+		jmp	.process
+
+		;--- '%%' ---;
+		.rep_pct:
+		mov	[printfBuff+r10], byte '%'
+		add	rdi, 2
+		add	r10, 1
+		add	r11, 1
+		jmp	.process
+		
+
+	.flushBuffer:
+	push	rdx
+	push	r11
+	mov	rax, NR_write
+	mov	rdi, FD_stdout
+	lea	rsi, [rel printfBuff]
+	mov	rdx, r11
+	syscall
+	pop	r10
+	pop	rdx
+	xor	r10, r10
+
+	cmp	r9b, 0
+	je	.flushReturn_0
+
+	.wrapup:
+	mov	rax, NR_write
+	mov	rdi, FD_stdout
+	lea	rsi, [rel printfBuff]
+	mov	rdx, r10
+	mov	r10, r11
+	syscall
+	mov	rax, r10
+	jmp	.quit
+
+	.emptyStr:
+	xor	rax, rax
+
+	.quit:
+	add	rsp, SIZE_QWORD
+	leave
+	ret
diff --git a/src/tests.asm b/src/tests.asm
index 0758fb3..e13e17f 100644
--- a/src/tests.asm
+++ b/src/tests.asm
@@ -5,6 +5,7 @@ extern	exit
 ; console.asm
 extern	print
 extern	puts
+extern	printf
 ; string.asm
 extern	strlen
 extern	strcpy
@@ -14,6 +15,7 @@ extern	strcat
 section .rodata
 	TEST_print		equ	1
 	TEST_puts		equ	1
+	TEST_printf		equ	1
 	TEST_strlen		equ	1
 	TEST_strcpy		equ	1
 	TEST_strcat		equ	1
@@ -30,6 +32,11 @@ section .rodata
 	; puts()
 	msgPuts			db	"TEST puts()",EOS
 
+	; printf()
+	msgPrintf		db	"TEST printf()",NL,EOS
+	printf1			db	TAB,"printf(",DQUO,"He%ll%o there%%%%%!%s%!%\n",DQUO,"): ",NL,TAB,TAB,EOS
+	printf1Str		db	"He%ll%o there%%%%%!%s%!%",NL,EOS
+
 	; strlen()
 	strlenStr1		db	"Hello",EOS
 	strlenStr2		db	"Hello, world!",NL,EOS
@@ -75,6 +82,20 @@ _start:
 	call	puts
 %endif
 
+;---
+;--- printf()
+;---
+%if TEST_printf
+	lea	rdi, [rel msgPrintf]
+	call	printf
+
+	; TEST 1
+	lea	rdi, [rel printf1]
+	call	print
+	lea	rdi, [rel printf1Str]
+	call	printf
+%endif
+
 ;---
 ;--- strlen()
 ;---