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292 lines
6.7 KiB
NASM
292 lines
6.7 KiB
NASM
; floppy
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;
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; Implement a block device around a TRS-80 floppy. It uses SVCs supplied by
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; TRS-DOS to do so.
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;
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; *** Floppy buffers ***
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;
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; The dual-buffer system is exactly the same as in the "sdc" module. See
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; comments there.
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;
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; *** Consts ***
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; Number of sector per cylinder. We only support single density for now.
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.equ FLOPPY_SEC_PER_CYL 10
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.equ FLOPPY_MAX_CYL 40
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.equ FLOPPY_BLKSIZE 256
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; *** Variables ***
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; This is a pointer to the currently selected buffer. This points to the BUFSEC
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; part, that is, two bytes before actual content begins.
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.equ FLOPPY_BUFPTR FLOPPY_RAMSTART
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; Sector number currently in FLOPPY_BUF1. Little endian like any other z80 word.
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.equ FLOPPY_BUFSEC1 @+2
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; Whether the buffer has been written to. 0 means clean. 1 means dirty.
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.equ FLOPPY_BUFDIRTY1 @+2
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; The contents of the buffer.
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.equ FLOPPY_BUF1 @+1
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; second buffer has the same structure as the first.
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.equ FLOPPY_BUFSEC2 @+FLOPPY_BLKSIZE
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.equ FLOPPY_BUFDIRTY2 @+2
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.equ FLOPPY_BUF2 @+1
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.equ FLOPPY_RAMEND @+FLOPPY_BLKSIZE
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; *** Code ***
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floppyInit:
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; Make sure that both buffers are flagged as invalid and not dirty
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xor a
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ld (FLOPPY_BUFDIRTY1), a
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ld (FLOPPY_BUFDIRTY2), a
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dec a
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ld (FLOPPY_BUFSEC1), a
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ld (FLOPPY_BUFSEC2), a
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ret
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; Returns whether D (cylinder) and E (sector) are in proper range.
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; Z for success.
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_floppyInRange:
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ld a, e
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cp FLOPPY_SEC_PER_CYL
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jp nc, unsetZ
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ld a, d
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cp FLOPPY_MAX_CYL
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jp nc, unsetZ
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xor a ; set Z
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ret
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; Read sector index specified in E and cylinder specified in D and place the
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; contents in buffer pointed to by (FLOPPY_BUFPTR).
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; If the operation is a success, updates buffer's sector to the value of DE.
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; Z on success
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floppyRdSec:
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call _floppyInRange
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ret nz
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push bc
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push hl
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ld a, 0x28 ; @DCSTAT
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ld c, 1 ; hardcoded to drive :1 for now
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rst 0x28
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jr nz, .end
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ld hl, (FLOPPY_BUFPTR) ; HL --> active buffer's sector
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ld (hl), e ; sector
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inc hl
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ld (hl), d ; cylinder
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inc hl ; dirty
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inc hl ; data
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ld a, 0x31 ; @RDSEC
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rst 0x28 ; sets proper Z
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.end:
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pop hl
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pop bc
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ret
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; Write the contents of buffer where (FLOPPY_BUFPTR) points to in sector
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; associated to it. Unsets the the buffer's dirty flag on success.
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; Z on success
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floppyWrSec:
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push ix
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ld ix, (FLOPPY_BUFPTR) ; IX points to sector
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xor a
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cp (ix+2) ; dirty flag
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pop ix
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ret z ; don't write if dirty flag is zero
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push hl
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push de
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push bc
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ld hl, (FLOPPY_BUFPTR) ; sector
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ld e, (hl)
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inc hl ; cylinder
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ld d, (hl)
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call _floppyInRange
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jr nz, .end
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ld c, 1 ; drive
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ld a, 0x28 ; @DCSTAT
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rst 0x28
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jr nz, .end
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inc hl ; dirty
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xor a
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ld (hl), a ; undirty the buffer
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inc hl ; data
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ld a, 0x35 ; @WRSEC
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rst 0x28 ; sets proper Z
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.end:
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pop bc
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pop de
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pop hl
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ret
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; Considering the first 15 bits of EHL, select the most appropriate of our two
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; buffers and, if necessary, sync that buffer with the floppy. If the selected
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; buffer doesn't have the same sector as what EHL asks, load that buffer from
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; the floppy.
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; If the dirty flag is set, we write the content of the in-memory buffer to the
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; floppy before we read a new sector.
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; Returns Z on success, NZ on error
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floppySync:
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push de
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; Given a 24-bit address in EHL, extracts the 16-bit sector from it and
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; place it in DE, following cylinder and sector rules.
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; EH is our sector index, L is our offset within the sector.
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ld d, e ; cylinder
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ld a, h ; sector
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; Let's process D first. Because our maximum number of sectors is 400
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; (40 * 10), D can only be either 0 or 1. If it's 1, we set D to 25 and
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; add 6 to A
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inc d \ dec d
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jr z, .loop1 ; skip
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ld d, 25
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add a, 6
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.loop1:
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cp FLOPPY_SEC_PER_CYL
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jr c, .loop1end
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sub FLOPPY_SEC_PER_CYL
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inc d
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jr .loop1
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.loop1end:
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ld e, a ; write final sector in E
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; Let's first see if our first buffer has our sector
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ld a, (FLOPPY_BUFSEC1) ; sector
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cp e
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jr nz, .notBuf1
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ld a, (FLOPPY_BUFSEC1+1) ; cylinder
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cp d
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jr z, .buf1Ok
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.notBuf1:
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; Ok, let's check for buf2 then
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ld a, (FLOPPY_BUFSEC2) ; sector
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cp e
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jr nz, .notBuf2
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ld a, (FLOPPY_BUFSEC2+1) ; cylinder
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cp d
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jr z, .buf2Ok
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.notBuf2:
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; None of our two buffers have the sector we need, we'll need to load
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; a new one.
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; We select our buffer depending on which is dirty. If both are on the
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; same status of dirtiness, we pick any (the first in our case). If one
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; of them is dirty, we pick the clean one.
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push de ; --> lvl 1
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ld de, FLOPPY_BUFSEC1
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ld a, (FLOPPY_BUFDIRTY1)
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or a ; is buf1 dirty?
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jr z, .ready ; no? good, that's our buffer
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; yes? then buf2 is our buffer.
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ld de, FLOPPY_BUFSEC2
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.ready:
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; At this point, DE points to one of our two buffers, the good one.
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; Let's save it to FLOPPY_BUFPTR
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ld (FLOPPY_BUFPTR), de
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pop de ; <-- lvl 1
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; We have to read a new sector, but first, let's write the current one
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; if needed.
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call floppyWrSec
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jr nz, .end ; error
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; Let's read our new sector in DE
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call floppyRdSec
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jr .end
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.buf1Ok:
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ld de, FLOPPY_BUFSEC1
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ld (FLOPPY_BUFPTR), de
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; Z already set
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jr .end
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.buf2Ok:
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ld de, FLOPPY_BUFSEC2
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ld (FLOPPY_BUFPTR), de
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; Z already set
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; to .end
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.end:
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pop de
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ret
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; Flush floppy buffers if dirty and then invalidates them.
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; We invalidate them so that we allow the case where we swap disks after a
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; flush. If we didn't invalidate the buffers, reading a swapped disk after a
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; flush would yield data from the previous disk.
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floppyFlush:
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ld hl, FLOPPY_BUFSEC1
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ld (FLOPPY_BUFPTR), hl
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call floppyWrSec
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ld hl, FLOPPY_BUFSEC2
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ld (FLOPPY_BUFPTR), hl
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call floppyWrSec
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call floppyInit
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xor a ; ensure Z
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ret
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; *** blkdev routines ***
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; Make HL point to its proper place in FLOPPY_BUF.
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; EHL currently is a 24-bit offset to read in the floppy. E=high byte,
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; HL=low word. Load the proper sector in memory and make HL point to the
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; correct data in the memory buffer.
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_floppyPlaceBuf:
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call floppySync
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ret nz ; error
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; At this point, we have the proper buffer in place and synced in
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; (FLOPPY_BUFPTR). Only L is important
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ld a, l
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ld hl, (FLOPPY_BUFPTR)
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inc hl ; sector MSB
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inc hl ; dirty flag
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inc hl ; contents
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; DE is now placed on the data part of the active buffer and all we need
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; is to increase DE by L.
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call addHL
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; Now, HL points exactly at the right byte in the active buffer.
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xor a ; ensure Z
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ret
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floppyGetB:
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push hl
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call _floppyPlaceBuf
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jr nz, .end ; NZ already set
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; This is it!
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ld a, (hl)
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cp a ; ensure Z
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.end:
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pop hl
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ret
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floppyPutB:
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push hl
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push af ; --> lvl 1. let's remember the char we put,
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; _floppyPlaceBuf destroys A.
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call _floppyPlaceBuf
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jr nz, .error
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; HL points to our dest. Recall A and write
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pop af ; <-- lvl 1
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ld (hl), a
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; Now, let's set the dirty flag
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ld a, 1
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ld hl, (FLOPPY_BUFPTR)
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inc hl ; sector MSB
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inc hl ; point to dirty flag
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ld (hl), a ; set dirty flag
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xor a ; ensure Z
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jr .end
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.error:
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; preserve error code
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ex af, af'
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pop af ; <-- lvl 1
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ex af, af'
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call unsetZ
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.end:
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pop hl
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ret
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