collapseos/parts/blockdev.asm

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NASM
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; 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.
; *** DEFINES ***
; BLOCKDEV_COUNT: The number of devices we manage.
; *** CONSTS ***
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BLOCKDEV_ERR_OUT_OF_BOUNDS .equ 0x03
BLOCKDEV_ERR_UNSUPPORTED .equ 0x04
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; *** VARIABLES ***
; Pointer to the selected block device. A block device is a 6 bytes block of
; memory with pointers to GetC, PutC and Seek routines, in that order. 0 means
; unsupported.
BLOCKDEV_SEL .equ BLOCKDEV_RAMSTART
BLOCKDEV_RAMEND .equ BLOCKDEV_SEL+2
; *** CODE ***
; Select block index specified in A
blkSel:
push af
push hl
ld hl, blkDevTbl
cp 0
jr z, .afterloop ; index is zero? don't loop
push bc
ld b, a
.loop:
ld a, 6
call addHL
djnz .loop
pop bc
.afterloop:
ld (BLOCKDEV_SEL), hl
pop Hl
pop af
ret
blkBselCmd:
.db "bsel", 0b001, 0, 0
ld a, (hl) ; argument supplied
cp BLOCKDEV_COUNT
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jr nc, .error ; if selection >= device count, error
call blkSel
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xor a
ret
.error:
ld a, BLOCKDEV_ERR_OUT_OF_BOUNDS
ret
; In those routines below, IY is destroyed (we don't push it to the stack). We
; seldom use it anyways...
; call routine in BLOCKDEV_SEL with offset IYL.
_blkCall:
push ix
push de
ld de, (BLOCKDEV_SEL)
; DE now points to the *address table*, not the routine addresses
; themselves. One layer of indirection left.
; slide by offset
push af
ld a, iyl
call addDE ; slide by offset
pop af
call intoDE
; Alright, now de points to what we want to call
ld ixh, d
ld ixl, e
pop de
; Before we call... is it zero? We don't want to call a zero.
push af
ld a, ixh
add a, ixl
jr c, .ok ; if there's a carry, it isn't zero
cp 0
jr z, .error ; if no carry and zero, then both numbers are
; zero
.ok:
pop af
call callIX
jr .end
.error:
pop af
ld a, BLOCKDEV_ERR_UNSUPPORTED
.end:
pop ix
ret
; Reads one character from selected device and returns its value in A. Always
; returns a character and waits until read if it has to.
blkGetC:
ld iyl, 0
jr _blkCall
; Writes character in A in current position in the selected device
blkPutC:
ld iyl, 2
jr _blkCall
blkSeekCmd:
.db "seek", 0b011, 0b001, 0
; HL points to two bytes that contain out address. Seek expects HL
; to directly contain that address.
ld a, (hl)
ex af, af'
inc hl
ld a, (hl)
ld l, a
ex af, af'
ld h, a
xor a
; Set position of selected device to the value specified in HL
blkSeek:
ld iyl, 4
jr _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 3 word addresses: GetC, PutC and Seek. An entry could look like:
; .dw mmapGetC, mmapPutC, mmapSeek
blkDevTbl: