From 68030372b72a538222e3f40deee6f7e40c0b4a1d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juhani=20Krekel=C3=A4?= <juhani@krekelä.fi>
Date: Sun, 26 Sep 2021 11:46:50 +0300
Subject: [PATCH] Load and execute command.com

---
 io.asm | 112 +++++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 97 insertions(+), 15 deletions(-)

diff --git a/io.asm b/io.asm
index 4bd50b9..f740efb 100644
--- a/io.asm
+++ b/io.asm
@@ -64,14 +64,26 @@ init:
 	shl ax, cl
 	; Memory size is passed in dx
 	mov dx, ax
+	call hexprint16
+	call newline
 
 	; Disk table is passed in si
 	mov si, offset iogroup:disks_table
 
 	call dosinit
 
-	; Memory for command.com is at ds, save for later and revert ds
-	push ds
+	; Memory for command.com is at ds, with PSP already set up
+
+	; Retrieve amount of memory in the program segment
+	mov cx, word ptr ds:6
+
+	; Set up DTA
+	mov dx, 100h
+	mov ah, 1ah
+	int 21h
+
+	; Save command.com's segment for later and revert ds for opening the FCB
+	mov bx, ds
 	mov ax, cs
 	mov ds, ax
 
@@ -79,15 +91,67 @@ init:
 	mov dx, offset iogroup:command_fcb
 	mov ah, 0fh
 	int 21h
-
 	test al, al
 	jnz open_error
 
 	mov al, '+'
-	jmp unfinished
+	mov ah, 0eh
+	int 10h
 
+	; Set random record field 0
+	mov word ptr command_fcb+33, 0
+	mov word ptr command_fcb+35, 0
+
+	; Set record size to 1 byte
+	mov word ptr command_fcb+14, 1
+
+	; Read command.com into memory
+	mov ah, 27h
+	int 21h
+
+	jcxz read_error
+
+	cmp al, 1
+	jne read_error
+
+	mov ax, cx
+	call hexprint16
+
+	mov al, '.'
+	mov ah, 0eh
+	int 10h
+
+	; Set up segments for command.com
+	mov ds, bx
+	mov es, bx
+	mov ss, bx
+	mov sp, 5ch ; Microsoft's documentation says to use this
+
+	; Put a zero at top of the stack
+	xor ax, ax
+	push ax
+
+	; Set default DTA
+	mov dx, 80h
+	mov ah, 1ah
+	int 21h
+
+	; Jump to command.com
+	push bx
+	mov ax, 100h
+	push ax
+
+trampoline proc far
+	ret
+trampoline endp
+
+	; TODO: Better error reporting
 	open_error:
 		mov al, '-'
+		jmp unfinished
+
+	read_error:
+		mov al, '!'
 
 	unfinished:
 		mov ah, 0eh
@@ -146,7 +210,24 @@ diskread proc far
 	int 10h
 	pop ax
 	call far_caller
+	push ax
+	call hexprint8
+	call space
+	mov ax, ds
+	call hexprint16
+	call space
+	mov ax, bx
+	call hexprint16
+	call space
+	mov ax, cx
+	call hexprint16
+	call space
+	mov ax, dx
+	call hexprint16
+	call newline
+	pop ax
 
+	; TODO: Everything except sregs can be trashed
 	push es
 	push ax
 	push bx
@@ -190,6 +271,11 @@ diskread proc far
 			jmp try_sector_read
 
 		sector_read_fail:
+			mov al, ah
+			call hexprint8;debg
+			mov al, '?'
+			mov ah, 0eh
+			int 10h
 			pop cx
 			pop ax
 
@@ -201,6 +287,7 @@ diskread proc far
 			pop es
 
 			mov al, 12 ; TODO: Don't hardcode
+			stc
 			ret
 
 		sector_read_success:
@@ -219,6 +306,7 @@ diskread proc far
 		pop ax
 		pop es
 
+		clc
 		ret
 
 diskread endp
@@ -256,10 +344,10 @@ chs proc
 
 	; cylinder (track)
 	mov ch, al
-	;shr ax, 1
-	;shr ax, 1
-	;and al, 0c0h
-	;or cl, al
+	shr ax, 1
+	shr ax, 1
+	and al, 0c0h
+	or cl, al
 
 	mov ax, dx
 	pop dx
@@ -280,13 +368,8 @@ diskwrite:
 ;  cf = 0 -> al = driver num
 ;  cf = 1 -> al = disk error code
 diskchange proc far
-	push ax
-	mov ax, 0e00h + 'd'
-	int 10h
-	pop ax
-	call far_caller
 	; TODO: Implement
-	mov ax, 0100h
+	mov ax, 0000h
 	clc
 	ret
 diskchange endp
@@ -320,7 +403,6 @@ flush endp
 
 mapdev proc far
 	; TODO: Implement
-	xor al, al
 	ret
 mapdev endp