From 187cd0fbd40764eb3bf88418952f07e74788180d Mon Sep 17 00:00:00 2001
From: mothcompute <mothcompute@protonmail.com>
Date: Wed, 2 Nov 2022 17:28:53 -0700
Subject: [PATCH] a couple bugfixes

---
 src/boot.s  | 17 +++++--------
 src/int21.s | 72 ++++++++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 63 insertions(+), 26 deletions(-)

diff --git a/src/boot.s b/src/boot.s
index eaac5a6..974e70e 100644
--- a/src/boot.s
+++ b/src/boot.s
@@ -1,33 +1,28 @@
-; foxdos boot sector. should load kernel into segment 0x50 at 0x00. max size
-; would be 64k but i doubt a dos needs more than that. stack is placed at
-; 0x10500. i would suggest changing this to load the kernel into extended
-; a20-gate memory so as not to overwrite the boot sector with larger kernels
-; but i will leave that for you to do. code is public domain as always. i
-; have not tested any of this so your mileage using it may vary
+; foxdos boot sector
 
 %include "config.s"
 
 [org 0x7C00]
 [bits 16]
 
-mov ax, K_ADDR << 4
+mov ax, K_ADDR >> 4
 push ax ; setup for ds
 mov es, ax
 mov gs, ax
 mov fs, ax
 
-xor bx, bx
-mov ds, bx
+xor cx, cx
+mov ds, cx
 mov word [0x21*4], 2 ; see kernel_entry
 mov word [0x21*4+2], ax
-pop ds
 
 mov ax, 0x200 | NUMSEG
-mov cl, 2
+mov cl, ah
 xor dh, dh
 int 13h
 
 ; 64kb stack
+pop ds
 mov ax, STACK_SEG
 mov ss, ax
 xor sp, sp
diff --git a/src/int21.s b/src/int21.s
index 75222d9..4538e74 100644
--- a/src/int21.s
+++ b/src/int21.s
@@ -2,14 +2,56 @@ int21:
 	push ds
 	push cs
 	pop ds
-	; your handler goes here
-	; you could maybe make a jump table that takes ah as input?
-	pop ds
+
+	; this probably works
+	mov word [tmp], bx ; cannot use stack for this
+	movzx bx, ah
+	shl bx, 1
+	add bx, fn
+	push end21 ; the proper return address
+	; generated return address. should avoid any issues
+	; with things like prefetch or instruction caches
+	push bx
+	mov bx, word [tmp]
+	ret
+
+end21:	pop ds
 	iret
 
+nul:	stc
+	ret
+
+; TODO figure out a way to format this table. maybe it can be autogenerated?
+; this table is misaligned for me and im the one who made it
+	;	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
+fn:	dw	nul,	rdin_echo,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 0
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 1
+	dw	nul,	nul,	nul,	nul,	nul,	setint,	nul,	nul,	nul,	nul,	nul,	nul,	gettime,	settime,	setverify,	nul	; 2
+	dw	getver,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 3
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	getret,	nul,	nul	; 4
+	dw	nul,	nul,	nul,	nul,	setverify,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 5
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 6
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 7
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 8
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; 9
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; A
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; B
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; C
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; D
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; E
+	dw	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul,	nul	; F
+
+tmp: dw 0
 retcode: dw 0
 verify: db 0
 
+rdin_echo:; ah=01h
+	push ax
+	mov ah, 8
+	int 0x21
+	; TODO does echo go to stdout or to screen?
+	pop ax
+	
 ; AH = 4dh
 ; get return code
 ; inputs:
@@ -17,11 +59,10 @@ verify: db 0
 ; outputs:
 ; AH: termination type (0 = normal, 1 = control-C abort, 2 = critical error abort, 3 = terminate and stay resident)
 ; AL: return code
-get_retcode:
-	mov ax, [retcode]
+getret:
 	xor ax, ax
-	mov [retcode], ax
-	retf
+	xchg [retcode], ax
+	ret
 
 ; AH = 30h
 ; get the DOS version number
@@ -30,11 +71,12 @@ get_retcode:
 ; outputs:
 ; AL: major version
 ; AH: minor version
-get_version:
+getver:
 	mov ax, 8 ; if it is not zero indexed this indicates windows ME
 	xor bx, bx ; update: what does that comment mean
 	mov cx, bx
-	retf
+	ret
+
 
 ; AH = 54h
 ; get disk verify flag
@@ -44,7 +86,7 @@ get_version:
 ; AL: 0 if off, 1 if on
 getverify:
 	mov al, [verify]
-	retf
+	ret
 
 ; AH = 2eh
 ; set disk verify flag
@@ -54,7 +96,7 @@ getverify:
 ; none
 setverify:
 	mov [verify], al
-	retf
+	ret
 
 ; AH = 35h
 ; get interrupt vector
@@ -76,7 +118,7 @@ getint:
 	mov word es, [di+2]
 	pop di
 	pop ds
-	retf
+	ret
 
 ; AH = 25h
 ; set interrupt vector
@@ -97,7 +139,7 @@ setint:
 	mov word [di+2], ds
 	pop ds
 	popa
-	retf
+	ret
 
 ; read from CMOS register
 ; inputs:
@@ -168,7 +210,7 @@ gettime:
 	add ch, 12
 
 .end:	pop ax
-	retf
+	ret
 
 ; AH = 2dh
 ; set system time in the CMOS
@@ -194,4 +236,4 @@ settime:
 	call wrcmos
 	xor al, al ; it probably succeeded, its fine. TODO: am i missing return codes anywhere else
 	pop bx
-	iret
+	ret