Implement date/time

The old code was essentially a no-op due to x86 taking shifts mod 32.
This lead DOS to believe that it had 10KiB of memory, which is not
enough to contain io.sys, the DOS kernel, and command.com. This causes
command.com to overwrite the DOS kernel when it goes to relocate itself
at the end of the memory, leading to system crash.

io.asm

@@ -22,7 +22,7 @@ data ends
 iogroup group code, constants, data
 
 code segment
-assume cs:iogroup, ds:iogroup, es:iogroup
+assume cs:iogroup
 org 0
 
 ; Jump table
@@ -36,9 +36,9 @@ jmp unimplemented ; 0012 Serial write
 jmp near ptr diskread ; 0015
 jmp diskwrite ; 0018
 jmp near ptr diskchange ; 001b
-jmp setdate ; 001e
-jmp settime ; 0021
-jmp gettime ; 0024
+jmp near ptr setdate ; 001e
+jmp near ptr settime ; 0021
+jmp near ptr gettime ; 0024
 jmp near ptr flush ; 0027
 jmp near ptr mapdev ; 002a
 
@@ -60,18 +60,30 @@ init:
 	; AX = memory size in kilobytes
 	; We want it in paragraphs
 	; There are 64 paragraphs in a kilobyte
-	mov cx, 64
-	shl al, cl
+	mov cx, 6
+	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 iogroup:command_fcb+33, 0
+	mov word ptr iogroup:command_fcb+35, 0
+
+	; Set record size to 1 byte
+	mov word ptr iogroup: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
@@ -238,7 +326,7 @@ chs proc
 	; cylinder = LBA / sectorspertrack / heads
 	; head = LBA / sectorspertrack % heads
 	; sector = LBA % sectorspertrack + 1
-	div cs:sectorspertrack
+	div iogroup:sectorspertrack
 	; ax = LBA / sectorspertrack
 	; dx = LBA % sectorspertrack
 
@@ -247,7 +335,7 @@ chs proc
 	inc cl
 
 	xor dx, dx
-	div cs:heads
+	div iogroup:heads
 	; ax = LBA / sectorspertrack / heads
 	; dx = LBA / sectorspertrack % heads
 
@@ -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
@@ -280,26 +368,138 @@ 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
 
-setdate:
-	mov al, 'd'
-	jmp error
-settime:
-	mov al, 't'
-	jmp error
-gettime:
-	mov al, 'T'
-	jmp error
+; IN:
+;  ax = days since 1980-01-01
+setdate proc far
+	mov iogroup:days_since_epoch, ax
+	ret
+setdate endp
+
+; IN:
+;  ch = hour
+;  cl = minute
+;  dh = second
+;  dl = 1/100 second
+settime proc far
+	push ax
+	push bx
+	push cx
+	push dx
+	push si
+	push di
+
+	; Save second & 1/100 second in bx
+	mov bx, dx
+
+	; Hour
+	mov al, ch
+	cbw
+	mul iogroup:doubleticks_per_hour
+	shl ax, 1
+	rcl dx, 1
+	; Store the partial sum in si:di
+	mov di, ax
+	mov si, dx
+
+	; Minute
+	mov al, cl
+	cbw
+	mul iogroup:ticks_per_minute
+	add di, ax
+	adc si, dx
+
+	; Second
+	mov al, bh
+	cbw
+	mul iogroup:ticks_per_second
+	add di, ax
+	adc si, dx
+
+	; 1/100 second
+	mov al, bl
+	cbw
+	div iogroup:cs_per_tick
+	xor ah, ah
+	add di, ax
+	adc si, 0
+
+	; Move result to cx:ds and set timer
+	mov cx, si
+	mov dx, di
+	mov ah, 01h
+	int 1ah
+
+	pop di
+	pop si
+	pop dx
+	pop cx
+	pop bx
+	pop ax
+	ret
+settime endp
+
+; OUT:
+;  ax = days since 1980-01-01
+;  ch = hour
+;  cl = minute
+;  dh = second
+;  dl = 1/100 second
+gettime proc far
+	; TODO: Use RTC if available
+	push bx
+
+	; Get ticks since midnight and day rollover flag
+	xor ah, ah
+	int 1ah
+
+	; al is nonzero if day has changed
+	test al, al
+	jz same_day
+	inc iogroup:days_since_epoch
+
+	same_day:
+
+	; Divide the ticks by 2, to keep all our arithmetic within bounds
+	shr cx, 1
+	rcr dx, 1
+
+	; int 1ah / ah=00 returns the ticks in cx:dx, div operates on dx:ax
+	mov ax, dx
+	mov dx, cx
+	div iogroup:doubleticks_per_hour
+	mov ch, al ; Hour
+
+	; Remainder in dx, range [0,doubleticks_per_hour[ represents an hour
+	; Multiply by 60, divide by doubleticks_per_hour to get the minute
+	mov ax, dx
+	mul iogroup:sixty
+	div iogroup:doubleticks_per_hour
+	mov cl, al ; Minute
+
+	; Do same again to get seconds
+	mov ax, dx
+	mul iogroup:sixty
+	div iogroup:doubleticks_per_hour
+	mov bl, al ; Save seconds in bl, since dh gets destroyed by next muldiv
+
+	; For 1/100th of a second, multiply by 100 instead
+	mov ax, dx
+	mul iogroup:hundred
+	div iogroup:doubleticks_per_hour
+	mov dh, bl ; Second
+	mov dl, al ; 1/100 second
+
+	mov ax, iogroup:days_since_epoch
+
+	pop bx
+	ret
+gettime endp
 
 flush proc far
 	push ax
@@ -320,7 +520,6 @@ flush endp
 
 mapdev proc far
 	; TODO: Implement
-	xor al, al
 	ret
 mapdev endp
 
@@ -362,6 +561,14 @@ parameters_320k:
 	dw 112 ; Number of directory entries
 	dw 320*2 ; Number of sectors
 
+doubleticks_per_hour dw 32772 ; 1800B0h / 2 / 24
+ticks_per_minute dw 1092 ; 1800B0h / 24 / 60
+ticks_per_second dw 18 ; 1800B0h / 24 / 60 / 60
+cs_per_tick db 5 ; 24 * 60 * 60 * 100 / 1800B0h
+
+sixty dw 60
+hundred dw 100
+
 constants ends
 
 data segment
@@ -371,6 +578,8 @@ command_fcb:
 	db "COMMAND COM"
 	db 25 dup (?)
 
+days_since_epoch: dw 0
+
 data ends
 
 code segment