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sdc: add layer of indirection to buffer system
Also, lay out the plan for adding a second buffer.
This commit is contained in:
Virgil Dupras
parent
b742d48b75
commit
db24e21276
162
kernel/sdc.asm
162
kernel/sdc.asm
@ -15,6 +15,39 @@
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; then placed on a buffer that can be read by reading the same port.
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;
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; It's through that kind of device that this code below is supposed to work.
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;
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; *** SDC buffers ***
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;
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; SD card's lowest common denominator in terms of block size is 512 bytes, so
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; that's what we deal with. To avoid wastefully reading entire blocks from the
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; card for one byte read ops, we buffer the last read block. If a GetC or PutC
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; operation is within that buffer, then no interaction with the SD card is
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; necessary.
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;
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; As soon as a GetC or PutC operation is made that is outside the current
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; buffer, we load a new block.
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;
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; When we PutC, we flag the buffer as "dirty". On the next buffer change (during
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; an out-of-buffer request or during an explicit "flush" operation), bytes
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; currently in the buffer will be written to the SD card.
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;
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; We hold 2 buffers in memory, each targeting a different sector and with its
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; own dirty flag. We do that to avoid wasteful block writing in the case where
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; we read data from a file in the SD card, process it and write the result
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; right away, in another file on the same card (zasm), on a different sector.
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;
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; If we only have one buffer in this scenario, we'll end up loading a new sector
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; at each GetC/PutC operation and, more importantly, writing a whole block for
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; a few bytes each time. This will wear the card prematurely (and be very slow).
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;
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; With 2 buffers, we solve the problem. Whenever GetC/PutC is called, we first
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; look if one of the buffer holds our sector. If not, we see if one of the
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; buffer is clean (not dirty). If yes, we use this one. If both are dirty or
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; clean, we use any. This way, as long as writing isn't made to random
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; addresses, we ensure that we don't write wastefully because read operations,
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; even if random, will always use the one buffer that isn't dirty.
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;
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; NOTE: the 2-buffers thing is still a work in progress...
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; *** Defines ***
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; SDC_PORT_CSHIGH: Port number to make CS high
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@ -25,15 +58,16 @@
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.equ SDC_BLKSIZE 512
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; *** Variables ***
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; Whenever we read a sector, we read a whole block at once and we store it
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; in memory. That's where it goes.
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.equ SDC_BUF SDC_RAMSTART
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; Sector number currently in SDC_BUF. 0xff, it's initial value, means "no
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; sector.
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.equ SDC_BUFSEC SDC_BUF+SDC_BLKSIZE
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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 SDC_BUFPTR SDC_RAMSTART
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; Sector number currently in SDC_BUF.
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.equ SDC_BUFSEC SDC_BUFPTR+2
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; Whether the buffer has been written to. 0 means clean. 1 means dirty.
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.equ SDC_BUFDIRTY SDC_BUFSEC+1
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.equ SDC_RAMEND SDC_BUFDIRTY+1
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; The contents of the buffer.
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.equ SDC_BUF SDC_BUFDIRTY+1
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.equ SDC_RAMEND SDC_BUF+SDC_BLKSIZE
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; *** Code ***
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; Wake the SD card up. After power up, a SD card has to receive at least 74
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@ -218,6 +252,8 @@ sdcInitialize:
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jr nz, .error
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; Success! out of idle mode!
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; initialize variables
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ld hl, SDC_BUFSEC
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ld (SDC_BUFPTR), hl
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ld a, 0xff
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ld (SDC_BUFSEC), a
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xor a
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@ -248,9 +284,10 @@ sdcSetBlkSize:
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pop hl
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ret
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; Read block index specified in A and place the contents in (SDC_BUF).
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; Doesn't check CRC. If the operation is a success, updates (SDC_BUFSEC) to the
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; value of A.
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; Read block index specified in A and place the contents in buffer pointed to
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; by (SDC_BUFPTR).
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; Doesn't check CRC. If the operation is a success, updates buffer's sector to
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; the value of A.
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; Returns 0 in A if success, non-zero if error.
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sdcReadBlk:
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push bc
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@ -281,7 +318,17 @@ sdcReadBlk:
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; We received our data token!
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; Data packets follow immediately, we have 512 of them to read
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ld bc, SDC_BLKSIZE
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ld hl, SDC_BUF
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ld hl, (SDC_BUFPTR) ; HL --> active buffer's sector
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; It sounds a bit wrong to set bufsec and dirty flag before we get our
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; actual data, but at this point, we don't have any error conditions
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; left, success is guaranteed. To avoid needlesssly INCing hl, let's
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; set sector and dirty along the way
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ld a, e ; sector number
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ld (hl), a
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inc hl ; dirty flag
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xor a ; unset
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ld (hl), a
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inc hl ; actual contents
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.loop2:
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call sdcIdle
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ld (hl), a
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@ -291,11 +338,7 @@ sdcReadBlk:
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; Read our 2 CRC bytes
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call sdcIdle
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call sdcIdle
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; success! Let's recall our orginal A arg and put it in SDC_BUFSEC
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ld a, e
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ld (SDC_BUFSEC), a
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xor a
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ld (SDC_BUFDIRTY), a
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; success!
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jr .end
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.error:
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; try to preserve error code
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@ -308,19 +351,25 @@ sdcReadBlk:
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pop bc
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ret
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; Write the contents of (SDC_BUF) in sector number (SDC_BUFSEC). Unsets the
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; (SDC_BUFDIRTY) flag on success.
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; Write the contents of buffer where (SDC_BUFPTR) points to in sector associated
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; to it. Unsets the the buffer's dirty flag on success.
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; A returns 0 in A on success (with Z set), non-zero (with Z unset) on error.
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sdcWriteBlk:
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ld a, (SDC_BUFDIRTY)
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or a ; cp 0
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ret z ; return success, but do nothing.
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push hl
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ld hl, (SDC_BUFPTR) ; HL points to sector
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inc hl ; now to dirty flag
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xor a
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cp (hl)
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jr z, .dontWrite ; A is already 0
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; At this point, HL points to dirty flag of the proper buffer
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push bc
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push hl
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push de
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out (SDC_PORT_CSLOW), a
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ld a, (SDC_BUFSEC)
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dec hl ; sector
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ld a, (hl)
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ld hl, 0 ; write single block at addr A
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ld d, 0
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ld e, a
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@ -340,7 +389,10 @@ sdcWriteBlk:
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; Sending our data token!
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ld bc, SDC_BLKSIZE
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ld hl, SDC_BUF
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ld hl, (SDC_BUFPTR)
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inc hl ; dirty flag
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inc hl ; beginning of contents
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.loop:
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ld a, (hl)
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call sdcSendRecv
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@ -359,8 +411,11 @@ sdcWriteBlk:
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; good! Now, we need to let the card process this data. It will return
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; 0xff when it's not busy any more.
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call sdcWaitResp
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; Success! Now let's unset the first flag
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ld hl, (SDC_BUFPTR)
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inc hl ; dirty flag
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xor a
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ld (SDC_BUFDIRTY), a
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ld (hl), a
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jr .end
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.error:
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; try to preserve error code
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@ -369,29 +424,31 @@ sdcWriteBlk:
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inc a ; zero, adjust
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.end:
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out (SDC_PORT_CSHIGH), a
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pop hl
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pop de
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pop bc
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.dontWrite:
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pop hl
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ret
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; Ensures that (SDC_BUFSEC) is in sync with HL, that is, that the current
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; buffer in memory corresponds to where HL points to. If it doesn't, loads
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; the sector that HL points to in (SDC_BUF) and update (SDC_BUFSEC).
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; If the (SDC_BUFDIRTY) flag is set, we write the content of the in-memory
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; buffer to the SD card before we read a new sector.
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; Ensures that the sector of the current buffer is in sync with HL, that is,
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; that the current buffer in memory corresponds to where HL points to in the SD
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; card. If it doesn't, loads the sector specified in the highest 7 bits of HL
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; in memory and update the buffer's sector.
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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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; SD card before we read a new sector.
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; Returns Z on success, not-Z on error (with the error code from either
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; sdcReadBlk or sdcWriteBlk)
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sdcSync:
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; HL points to the character we're supposed to read or right now,
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; but we first have to check whether we need to load a new sector in
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; memory. To do this, we compare the high 7 bits of HL with
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; (SDC_BUFSEC). If they're different, we need to load a new block.
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push hl
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ld a, (SDC_BUFSEC)
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ld l, a
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; the buffer's sector. If they're different, we need to load a new block.
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ld a, h
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srl a
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cp l
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pop hl
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srl a ; A --> the requested sector number
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push hl ; <|
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ld hl, (SDC_BUFPTR) ; | HL points to sector number
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cp (hl) ; |
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pop hl ; <|
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ret z ; equal? nothing to do
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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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@ -423,22 +480,25 @@ sdcFlushCmd:
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_sdcPlaceBuf:
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call sdcSync
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ret nz ; error
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push de
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ld de, (SDC_BUFPTR)
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inc de ; dirty flag
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inc de ; contents
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ld a, h ; high byte
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and 0x01 ; is first bit set?
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ld a, l ; doesn't change flags
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jr nz, .highbuf ; first bit set? we're in the "highbuf" zone.
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; lowbuf zone
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; Read byte from memory at proper offset in lowbuf (first 0x100 bytes)
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ld hl, SDC_BUF
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jr .read
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.highbuf:
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; Read byte from memory at proper offset in highbuf (0x100-0x1ff)
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ld hl, SDC_BUF+0x100
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jr z, .read ; first bit reset? we're in the "lowbuf" zone.
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; DE already points to the right place.
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; We're in the highbuf zone, let's inc DE by 0x100, which, as it turns
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; out, is quite easy.
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inc d
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.read:
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; HL is now placed either on the lower or higher half of SDC_BUF and
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; all we need is to increase HL by the number in A which is the lower
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; half of our former HL value.
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call addHL
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; DE is now placed either on the lower or higher half of the active
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; buffer and all we need is to increase DE the lower half of HL.
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ld a, l
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call addDE
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ex de, hl
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pop de
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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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