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291 lines
6.1 KiB
NASM
291 lines
6.1 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 module exposes a seek/tell/getc/putc API that is then re-routed to
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; underlying drivers. There will eventually be more than one driver type, but
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; for now we sit on only one type of driver: random access driver.
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;
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; *** Random access drivers ***
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;
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; Random access drivers are expected to supply two routines: GetC and PutC.
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;
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; GetC:
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; Reads one character at address specified in DE/HL and returns its value in A.
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; Sets Z according to whether read was successful: Set if successful, unset
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; if not.
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;
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; Unsuccessful reads generally mean that requested addr is out of bounds (we
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; reached EOF).
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;
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; PutC:
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; Writes character in A at address specified in DE/HL. Sets Z according to
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; whether the operation was successful.
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;
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; Unsuccessful writes generally mean that we're out of bounds for writing.
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;
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; All routines are expected to preserve unused registers except IX which is
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; explicitly protected during GetC/PutC calls. This makes quick "handle+jump"
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; definitions possible.
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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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.equ BLOCKDEV_SEEK_ABSOLUTE 0
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.equ BLOCKDEV_SEEK_FORWARD 1
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.equ BLOCKDEV_SEEK_BACKWARD 2
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.equ BLOCKDEV_SEEK_BEGINNING 3
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.equ BLOCKDEV_SEEK_END 4
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.equ BLOCKDEV_SIZE 8
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; *** VARIABLES ***
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; Pointer to the selected block device. A block device is a 8 bytes block of
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; memory with pointers to GetC, PutC, and a 32-bit counter, in that order.
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.equ BLOCKDEV_SEL BLOCKDEV_RAMSTART
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.equ BLOCKDEV_RAMEND BLOCKDEV_SEL+BLOCKDEV_SIZE
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; *** CODE ***
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; Select block index specified in A and place them in routine pointers at (DE).
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; For example, for a "regular" blkSel, you will want to set DE to BLOCKDEV_SEL.
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blkSel:
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push af
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push de
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push hl
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ld hl, blkDevTbl
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or a ; cp 0
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jr z, .end ; 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, 4 ; |
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call addHL ; |
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djnz .loop ; |
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pop bc ; <|
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.end:
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call blkSet
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pop hl
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pop de
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pop af
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ret
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; Setup blkdev handle in (DE) using routines at (HL).
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blkSet:
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push af
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push de
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push hl
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; Write GETC
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push hl ; <|
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call intoHL ; |
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call writeHLinDE ; |
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inc de ; |
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inc de ; |
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pop hl ; <|
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inc hl
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inc hl
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; Write PUTC
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call intoHL
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call writeHLinDE
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inc de
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inc de
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; Initialize pos
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xor a
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ld (de), a
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inc de
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ld (de), a
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inc de
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ld (de), a
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inc de
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ld (de), a
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pop hl
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pop de
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pop af
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ret
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_blkInc:
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ret nz ; don't advance when in error condition
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push af
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push hl
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ld a, BLOCKDEV_SEEK_FORWARD
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ld hl, 1
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call _blkSeek
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pop hl
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pop af
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ret
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; Reads one character from selected device and returns its value in A.
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; Sets Z according to whether read was successful: Set if successful, unset
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; if not.
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blkGetC:
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ld ix, BLOCKDEV_SEL
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_blkGetC:
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push hl
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push de
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call _blkTell
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call callIXI
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pop de
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pop hl
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jr _blkInc ; advance and return
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; Writes character in A in current position in the selected device. Sets Z
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; according to whether the operation was successful.
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blkPutC:
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ld ix, BLOCKDEV_SEL
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_blkPutC:
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push ix
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push hl
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push de
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call _blkTell
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inc ix ; make IX point to PutC
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inc ix
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call callIXI
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pop de
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pop hl
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pop ix
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jr _blkInc ; advance and return
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; Reads B chars from blkGetC and copy them in (HL).
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; Sets Z if successful, unset Z if there was an error.
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blkRead:
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ld ix, BLOCKDEV_SEL
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_blkRead:
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push hl
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push bc
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.loop:
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call _blkGetC
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jr nz, .end ; Z already unset
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ld (hl), a
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inc hl
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djnz .loop
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cp a ; ensure Z
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.end:
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pop bc
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pop hl
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ret
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; Writes B chars to blkPutC from (HL).
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; Sets Z if successful, unset Z if there was an error.
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blkWrite:
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ld ix, BLOCKDEV_SEL
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_blkWrite:
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push hl
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push bc
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.loop:
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ld a, (hl)
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call _blkPutC
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jr nz, .end ; Z already unset
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inc hl
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djnz .loop
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cp a ; ensure Z
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.end:
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pop bc
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pop hl
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ret
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; Seeks the block device in one of 5 modes, which is the A argument:
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; 0 : Move exactly to X, X being the HL/DE argument.
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; 1 : Move forward by X bytes, X being the HL argument (no DE)
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; 2 : Move backwards by X bytes, X being the HL argument (no DE)
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; 3 : Move to the end
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; 4 : Move to the beginning
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; Set position of selected device to the value specified in HL (low) and DE
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; (high). DE is only used for mode 0.
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;
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; When seeking to an out-of-bounds position, the resulting position will be
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; one position ahead of the last valid position. Therefore, GetC after a seek
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; to end would always fail.
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;
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; If the device is "growable", it's possible that seeking to end when calling
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; PutC doesn't necessarily result in a failure.
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blkSeek:
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ld ix, BLOCKDEV_SEL
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_blkSeek:
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cp BLOCKDEV_SEEK_FORWARD
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jr z, .forward
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cp BLOCKDEV_SEEK_BACKWARD
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jr z, .backward
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cp BLOCKDEV_SEEK_BEGINNING
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jr z, .beginning
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cp BLOCKDEV_SEEK_END
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jr z, .end
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; all other modes are considered absolute
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ld (ix+4), e
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ld (ix+5), d
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ld (ix+6), l
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ld (ix+7), h
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ret
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.forward:
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push bc ; <-|
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push hl ; <||
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ld l, (ix+6) ; || low byte
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ld h, (ix+7) ; ||
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pop bc ; <||
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add hl, bc ; |
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pop bc ; <-|
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ld (ix+6), l
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ld (ix+7), h
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ret nc ; no carry? no need to adjust high byte
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; carry, adjust high byte
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inc (ix+4)
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ret nz
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inc (ix+5)
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ret
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.backward:
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and a ; clear carry
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push bc ; <-|
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push hl ; <||
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ld l, (ix+6) ; || low byte
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ld h, (ix+7) ; ||
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pop bc ; <||
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sbc hl, bc ; |
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pop bc ; <-|
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ld (ix+6), l
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ld (ix+7), h
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ret nc ; no carry? no need to adjust high byte
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ld a, 0xff
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dec (ix+4)
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cp (ix+4)
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ret nz
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; we decremented from 0
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dec (ix+5)
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ret
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.beginning:
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xor a
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ld (ix+4), a
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ld (ix+5), a
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ld (ix+6), a
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ld (ix+7), a
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ret
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.end:
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ld a, 0xff
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ld (ix+4), a
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ld (ix+5), a
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ld (ix+6), a
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ld (ix+7), a
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ret
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; Returns the current position of the selected device in HL (low) and DE (high).
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blkTell:
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ld ix, BLOCKDEV_SEL
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_blkTell:
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ld e, (ix+4)
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ld d, (ix+5)
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ld l, (ix+6)
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ld h, (ix+7)
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ret
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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 4 word addresses: GetC, PutC and Seek, Tell. An entry could look like:
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; .dw mmapGetC, mmapPutC, mmapSeek, mmapTell
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blkDevTbl:
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