collapseos/kernel/blockdev.asm
Virgil Dupras 7fad3b0c90 Move /parts/z80 to /kernel
Let go of that "meta os" thing. it's not as meta as I made it sound
like. It's a kernel.
2019-05-19 11:19:41 -04:00

259 lines
5.9 KiB
NASM

; blockdev
;
; A block device is an abstraction over something we can read from, write to.
;
; A device that fits this abstraction puts the properly hook into itself, and
; then the glue code assigns a blockdev ID to that device. It then becomes easy
; to access arbitrary devices in a convenient manner.
;
; This part exposes a new "bsel" command to select the currently active block
; device.
;
; *** Blockdev routines ***
;
; There are 4 blockdev routines that can be defined by would-be block devices
; and they follow these specifications:
;
; GetC:
; Reads one character from selected device and returns its value in A.
; Sets Z according to whether read was successful: Set if successful, unset
; if not.
;
; A successful GetC should advance the "pointer" of the device (if there is one)
; by one byte so that a subsequent GetC will read the next char. Unsuccessful
; reads generally mean that we reached EOF.
;
;
; PutC:
; Writes character in A in current position in the selected device. Sets Z
; according to whether the operation was successful.
;
; A successful PutC should advance the "pointer" of the device (if there is one)
; by one byte so that the next PutC places the next char next to this one.
; Unsuccessful writes generally mean that we reached EOF.
;
; Seek:
; Place device "pointer" at position dictated by HL.
;
; Tell:
; Return the position of the "pointer" in HL
;
; All routines are expected to preserve unused registers.
; *** DEFINES ***
; BLOCKDEV_COUNT: The number of devices we manage.
; *** CONSTS ***
.equ BLOCKDEV_ERR_OUT_OF_BOUNDS 0x03
.equ BLOCKDEV_ERR_UNSUPPORTED 0x04
.equ BLOCKDEV_SEEK_ABSOLUTE 0
.equ BLOCKDEV_SEEK_FORWARD 1
.equ BLOCKDEV_SEEK_BACKWARD 2
.equ BLOCKDEV_SEEK_BEGINNING 3
.equ BLOCKDEV_SEEK_END 4
; *** VARIABLES ***
; Pointer to the selected block device. A block device is a 8 bytes block of
; memory with pointers to GetC, PutC, Seek and Tell routines, in that order.
; 0 means unsupported.
.equ BLOCKDEV_GETC BLOCKDEV_RAMSTART
.equ BLOCKDEV_PUTC BLOCKDEV_GETC+2
.equ BLOCKDEV_SEEK BLOCKDEV_PUTC+2
.equ BLOCKDEV_TELL BLOCKDEV_SEEK+2
.equ BLOCKDEV_RAMEND BLOCKDEV_TELL+2
; *** CODE ***
; Select block index specified in A and place them in routine pointers at (DE).
; For example, for a "regular" blkSel, you will want to set DE to BLOCKDEV_GETC.
blkSel:
push af
push de
push hl
ld hl, blkDevTbl
or a ; cp 0
jr z, .afterloop ; index is zero? don't loop
push bc
ld b, a
.loop:
ld a, 8
call addHL
djnz .loop
pop bc
.afterloop:
push hl
call intoHL
call writeHLinDE
inc de
inc de
pop hl
inc hl
inc hl
push hl
call intoHL
call writeHLinDE
inc de
inc de
pop hl
inc hl
inc hl
push hl
call intoHL
call writeHLinDE
inc de
inc de
pop hl
inc hl
inc hl
call intoHL
call writeHLinDE
pop hl
pop de
pop af
ret
; call IX unless it's zero
_blkCall:
; Before we call... is IX zero? We don't want to call a zero.
push af
xor a
push hl
push ix \ pop hl
cp h
jr nz, .ok ; not zero, ok
cp l
jr z, .error ; zero, error
.ok:
pop hl
pop af
call callIX
ret
.error:
pop hl
pop af
ld a, BLOCKDEV_ERR_UNSUPPORTED
ret
; Reads one character from selected device and returns its value in A.
; Sets Z according to whether read was successful: Set if successful, unset
; if not.
blkGetC:
ld ix, (BLOCKDEV_GETC)
jr _blkCall
; Repeatedly call blkGetC until the call is a success.
blkGetCW:
ld ix, (BLOCKDEV_GETC)
.loop:
call callIX
jr nz, .loop
ret
; Reads B chars from blkGetC and copy them in (HL).
; Sets Z if successful, unset Z if there was an error.
blkRead:
ld ix, (BLOCKDEV_GETC)
_blkRead:
push hl
push bc
.loop:
call _blkCall
jr nz, .end ; Z already unset
ld (hl), a
inc hl
djnz .loop
cp a ; ensure Z
.end:
pop bc
pop hl
ret
; Writes character in A in current position in the selected device. Sets Z
; according to whether the operation was successful.
blkPutC:
ld ix, (BLOCKDEV_PUTC)
jr _blkCall
; Writes B chars to blkPutC from (HL).
; Sets Z if successful, unset Z if there was an error.
blkWrite:
ld ix, (BLOCKDEV_PUTC)
_blkWrite:
push hl
push bc
.loop:
ld a, (hl)
call _blkCall
jr nz, .end ; Z already unset
inc hl
djnz .loop
cp a ; ensure Z
.end:
pop bc
pop hl
ret
; Seeks the block device in one of 5 modes, which is the A argument:
; 0 : Move exactly to X, X being the HL argument.
; 1 : Move forward by X bytes, X being the HL argument
; 2 : Move backwards by X bytes, X being the HL argument
; 3 : Move to the end
; 4 : Move to the beginning
; Set position of selected device to the value specified in HL
;
; When seeking to an out-of-bounds position, the resulting position will be
; one position ahead of the last valid position. Therefore, GetC after a seek
; to end would always fail.
;
; If the device is "growable", it's possible that seeking to end when calling
; PutC doesn't necessarily result in a failure.
blkSeek:
ld ix, (BLOCKDEV_SEEK)
ld iy, (BLOCKDEV_TELL)
_blkSeek:
push de
cp BLOCKDEV_SEEK_FORWARD
jr z, .forward
cp BLOCKDEV_SEEK_BACKWARD
jr z, .backward
cp BLOCKDEV_SEEK_BEGINNING
jr z, .beginning
cp BLOCKDEV_SEEK_END
jr z, .end
; all other modes are considered absolute
jr .seek ; for absolute mode, HL is already correct
.forward:
ex de, hl ; DE has our offset
; We want to be able to plug our own TELL function, which is why we
; don't call blkTell directly here.
; Calling TELL
call callIY ; HL has our curpos
add hl, de
jr nc, .seek ; no carry? alright!
; we have carry? out of bounds, set to maximum
.backward:
; TODO - subtraction are more complicated...
jr .seek
.beginning:
ld hl, 0
jr .seek
.end:
ld hl, 0xffff
.seek:
pop de
jp _blkCall
; Returns the current position of the selected device in HL.
blkTell:
ld ix, (BLOCKDEV_TELL)
jp _blkCall
; This label is at the end of the file on purpose: the glue file should include
; a list of device routine table entries just after the include. Each line
; has 4 word addresses: GetC, PutC and Seek, Tell. An entry could look like:
; .dw mmapGetC, mmapPutC, mmapSeek, mmapTell
blkDevTbl: