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21eb64f751
The approach used for shell extra commands also works for blockdev. Much simpler and tighter this way.
133 lines
2.9 KiB
NASM
133 lines
2.9 KiB
NASM
; blockdev
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;
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; A block device is an abstraction over something we can read from, write to.
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;
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; A device that fits this abstraction puts the properly hook into itself, and
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; then the glue code assigns a blockdev ID to that device. It then becomes easy
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; to access arbitrary devices in a convenient manner.
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;
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; This part exposes a new "bsel" command to select the currently active block
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; device.
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; *** DEFINES ***
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; BLOCKDEV_COUNT: The number of devices we manage.
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; *** CONSTS ***
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BLOCKDEV_ERR_OUT_OF_BOUNDS .equ 0x03
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BLOCKDEV_ERR_UNSUPPORTED .equ 0x04
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; *** VARIABLES ***
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; Pointer to the selected block device. A block device is a 6 bytes block of
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; memory with pointers to GetC, PutC and Seek routines, in that order. 0 means
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; unsupported.
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BLOCKDEV_SEL .equ BLOCKDEV_RAMSTART
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BLOCKDEV_RAMEND .equ BLOCKDEV_SEL+2
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; *** CODE ***
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; Select block index specified in A
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blkSel:
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push af
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push hl
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ld hl, blkDevTbl
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cp 0
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jr z, .afterloop ; index is zero? don't loop
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push bc
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ld b, a
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.loop:
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ld a, 6
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call addHL
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djnz .loop
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pop bc
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.afterloop:
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ld (BLOCKDEV_SEL), hl
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pop Hl
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pop af
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ret
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blkBselCmd:
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.db "bsel", 0b001, 0, 0
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ld a, (hl) ; argument supplied
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cp BLOCKDEV_COUNT
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jr nc, .error ; if selection >= device count, error
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call blkSel
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xor a
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ret
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.error:
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ld a, BLOCKDEV_ERR_OUT_OF_BOUNDS
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ret
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; In those routines below, IY is destroyed (we don't push it to the stack). We
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; seldom use it anyways...
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; call routine in BLOCKDEV_SEL with offset IYL.
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_blkCall:
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push ix
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push de
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ld de, (BLOCKDEV_SEL)
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; DE now points to the *address table*, not the routine addresses
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; themselves. One layer of indirection left.
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; slide by offset
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push af
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ld a, iyl
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call addDE ; slide by offset
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pop af
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call intoDE
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; Alright, now de points to what we want to call
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ld ixh, d
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ld ixl, e
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pop de
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; Before we call... is it zero? We don't want to call a zero.
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push af
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ld a, ixh
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add a, ixl
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jr c, .ok ; if there's a carry, it isn't zero
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cp 0
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jr z, .error ; if no carry and zero, then both numbers are
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; zero
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.ok:
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pop af
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call callIX
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jr .end
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.error:
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pop af
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ld a, BLOCKDEV_ERR_UNSUPPORTED
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.end:
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pop ix
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ret
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; Reads one character from selected device and returns its value in A. Always
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; returns a character and waits until read if it has to.
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blkGetC:
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ld iyl, 0
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jr _blkCall
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; Writes character in A in current position in the selected device
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blkPutC:
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ld iyl, 2
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jr _blkCall
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blkSeekCmd:
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.db "seek", 0b011, 0b001, 0
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; HL points to two bytes that contain out address. Seek expects HL
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; to directly contain that address.
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ld a, (hl)
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ex af, af'
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inc hl
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ld a, (hl)
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ld l, a
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ex af, af'
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ld h, a
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xor a
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; Set position of selected device to the value specified in HL
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blkSeek:
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ld iyl, 4
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jr _blkCall
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; This label is at the end of the file on purpose: the glue file should include
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; a list of device routine table entries just after the include. Each line
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; has 3 word addresses: GetC, PutC and Seek. An entry could look like:
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; .dw mmapGetC, mmapPutC, mmapSeek
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blkDevTbl:
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