collapseos/kernel/sdc.asm

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; sdc
;
; Manages the initialization of a SD card and implement a block device to read
; and write from/to it, in SPI mode.
;
; Note that SPI can't really be used directly from the z80, so this part
; assumes that you have a device that handles SPI communication on behalf of
; the z80. This device is assumed to work in a particular way. See the
; "rc2014/sdcard" recipe for details.
;
; That device has 3 ports. One write-only port to make CS high, one to make CS
; low (data sent is irrelevant), and one read/write port to send and receive
; bytes with the card through the SPI protocol. The device acts as a SPI master
; and writing to that port initiates a byte exchange. Data from the slave is
; then placed on a buffer that can be read by reading the same port.
;
; It's through that kind of device that this code below is supposed to work.
;
; *** SDC buffers ***
;
; SD card's lowest common denominator in terms of block size is 512 bytes, so
; that's what we deal with. To avoid wastefully reading entire blocks from the
; card for one byte read ops, we buffer the last read block. If a GetB or PutB
; operation is within that buffer, then no interaction with the SD card is
; necessary.
;
; As soon as a GetB or PutB operation is made that is outside the current
; buffer, we load a new block.
;
; When we PutB, we flag the buffer as "dirty". On the next buffer change (during
; an out-of-buffer request or during an explicit "flush" operation), bytes
; currently in the buffer will be written to the SD card.
;
; We hold 2 buffers in memory, each targeting a different sector and with its
; own dirty flag. We do that to avoid wasteful block writing in the case where
; we read data from a file in the SD card, process it and write the result
; right away, in another file on the same card (zasm), on a different sector.
;
; If we only have one buffer in this scenario, we'll end up loading a new sector
; at each GetB/PutB operation and, more importantly, writing a whole block for
; a few bytes each time. This will wear the card prematurely (and be very slow).
;
; With 2 buffers, we solve the problem. Whenever GetB/PutB is called, we first
; look if one of the buffer holds our sector. If not, we see if one of the
; buffer is clean (not dirty). If yes, we use this one. If both are dirty or
; clean, we use any. This way, as long as writing isn't made to random
; addresses, we ensure that we don't write wastefully because read operations,
; even if random, will always use the one buffer that isn't dirty.
; *** Defines ***
; SDC_PORT_CSHIGH: Port number to make CS high
; SDC_PORT_CSLOW: Port number to make CS low
; SDC_PORT_SPI: Port number to send/receive SPI data
; *** Consts ***
.equ SDC_BLKSIZE 512
.equ SDC_MAXTRIES 8
; *** Variables ***
; This is a pointer to the currently selected buffer. This points to the BUFSEC
; part, that is, two bytes before actual content begins.
.equ SDC_BUFPTR SDC_RAMSTART
; Count the number of times we tried a particular read or write operation. When
; CRC check fail, we retry. After SDC_MAXTRIES failures, we stop.
.equ SDC_RETRYCNT SDC_BUFPTR+2
; Sector number currently in SDC_BUF1. Little endian like any other z80 word.
.equ SDC_BUFSEC1 SDC_RETRYCNT+1
; Whether the buffer has been written to. 0 means clean. 1 means dirty.
.equ SDC_BUFDIRTY1 SDC_BUFSEC1+2
; The contents of the buffer.
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.equ SDC_BUF1 SDC_BUFDIRTY1+1
; CRC bytes for the buffer. They're placed after the contents because that makes
; things easier processing-wise. Because the SD card sends them right after the
; contents, all we need to do is read SDC_BLKSIZE+2.
; IMPORTANT NOTE: This is big endian. The SD card sends the MSB first, so we
; keep it in memory this way.
.equ SDC_CRC1 SDC_BUF1+SDC_BLKSIZE
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; second buffer has the same structure as the first.
.equ SDC_BUFSEC2 SDC_CRC1+2
.equ SDC_BUFDIRTY2 SDC_BUFSEC2+2
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.equ SDC_BUF2 SDC_BUFDIRTY2+1
.equ SDC_CRC2 SDC_BUF2+SDC_BLKSIZE
.equ SDC_RAMEND SDC_CRC2+2
; *** Code ***
; Wake the SD card up. After power up, a SD card has to receive at least 74
; dummy clocks with CS and DI high. We send 80.
sdcWakeUp:
out (SDC_PORT_CSHIGH), a
ld b, 10 ; 10 * 8 == 80
ld a, 0xff
.loop:
out (SDC_PORT_SPI), a
nop
djnz .loop
ret
; Initiate SPI exchange with the SD card. A is the data to send. Received data
; is placed in A.
sdcSendRecv:
out (SDC_PORT_SPI), a
nop
nop
in a, (SDC_PORT_SPI)
nop
nop
ret
sdcIdle:
ld a, 0xff
jp sdcSendRecv
; sdcSendRecv 0xff until the response is something else than 0xff for a maximum
; of 20 times. Returns 0xff if no response.
sdcWaitResp:
push bc
ld b, 20
.loop:
call sdcIdle
inc a ; if 0xff, it's going to become zero
jr nz, .end ; not zero? good, that's our command
djnz .loop
.end:
; whether we had a success or failure, we return the result.
; But first, let's bring it back to its original value.
dec a
pop bc
ret
; The opposite of sdcWaitResp: we wait until response if 0xff. After a
; successful read or write operation, the card will be busy for a while. We need
; to give it time before interacting with it again. Technically, we could
; continue processing on our side while the card it busy, and maybe we will one
; day, but at the moment, I'm having random write errors if I don't do this
; right after a write, so I prefer to stay cautious for now.
; This has no error condition and preserves A
sdcWaitReady:
push af
; for now, we have no timeout for waiting. It means that broken SD
; cards can cause infinite loops.
.loop:
call sdcIdle
inc a ; if 0xff, it's going to become zero
jr nz, .loop ; not zero? still busy. loop
pop af
ret
; Sends a command to the SD card, along with arguments and specified CRC fields.
; (CRC is only needed in initial commands though).
; A: Command to send
; H: Arg 1 (MSB)
; L: Arg 2
; D: Arg 3
; E: Arg 4 (LSB)
;
; Returns R1 response in A.
;
; This does *not* handle CS. You have to select/deselect the card outside this
; routine.
sdcCmd:
push bc
; Wait until ready to receive commands
push af
call sdcWaitResp
pop af
ld c, 0 ; init CRC
call .crc7
call sdcSendRecv
; Arguments
ld a, h
call .crc7
call sdcSendRecv
ld a, l
call .crc7
call sdcSendRecv
ld a, d
call .crc7
call sdcSendRecv
ld a, e
call .crc7
call sdcSendRecv
; send CRC
ld a, c
or 0x01 ; ensure stop bit is set
call sdcSendRecv
; And now we just have to wait for a valid response...
call sdcWaitResp
pop bc
ret
; push A into C and compute CRC7 with 0x09 polynomial
; Note that the result is "left aligned", that is, that 8th bit to the "right"
; is insignificant (will be stop bit).
.crc7:
push af
xor c
ld b, 8
.loop:
sla a
jr nc, .noCRC
; msb was set, apply polynomial
xor 0x12 ; 0x09 << 1. We apply CRC on high 7 bits
.noCRC:
djnz .loop
ld c, a
pop af
ret
; Send a command that expects a R1 response, handling CS.
sdcCmdR1:
out (SDC_PORT_CSLOW), a
call sdcCmd
out (SDC_PORT_CSHIGH), a
ret
; Send a command that expects a R7 response, handling CS. A R7 is a R1 followed
; by 4 bytes. Those 4 bytes are returned in HL/DE in the same order as in
; sdcCmd.
sdcCmdR7:
out (SDC_PORT_CSLOW), a
call sdcCmd
; We have our R1 response in A. Let's try reading the next 4 bytes in
; case we have a R3.
push af
call sdcIdle
ld h, a
call sdcIdle
ld l, a
call sdcIdle
ld d, a
call sdcIdle
ld e, a
pop af
out (SDC_PORT_CSHIGH), a
ret
; Initialize a SD card. This should be called at least 1ms after the powering
; up of the card. Sets result code in A. Zero means success, non-zero means
; error.
sdcInitialize:
push hl
push de
push bc
call sdcWakeUp
; Call CMD0 and expect a 0x01 response (card idle)
; This should be called multiple times. We're actually expected to.
; Let's call this for a maximum of 10 times.
ld b, 10
.loop1:
ld a, 0b01000000 ; CMD0
ld hl, 0
ld de, 0
call sdcCmdR1
cp 0x01
jp z, .cmd0ok
djnz .loop1
; Nothing? error
jr .error
.cmd0ok:
; Then comes the CMD8. We send it with a 0x01aa argument and expect
; a 0x01aa argument back, along with a 0x01 R1 response.
ld a, 0b01001000 ; CMD8
ld hl, 0
ld de, 0x01aa
call sdcCmdR7
cp 0x01
jr nz, .error
xor a
cp h ; H is zero
jr nz, .error
cp l ; L is zero
jr nz, .error
ld a, d
cp 0x01
jp nz, .error
ld a, e
cp 0xaa
jr nz, .error
; Now we need to repeatedly run CMD55+CMD41 (0x40000000) until we
; the card goes out of idle mode, that is, when it stops sending us
; 0x01 response and send us 0x00 instead. Any other response means that
; initialization failed.
.loop2:
ld a, 0b01110111 ; CMD55
ld hl, 0
ld de, 0
call sdcCmdR1
cp 0x01
jr nz, .error
ld a, 0b01101001 ; CMD41 (0x40000000)
ld hl, 0x4000
ld de, 0x0000
call sdcCmdR1
cp 0x01
jr z, .loop2
or a ; cp 0
jr nz, .error
; Success! out of idle mode!
jr .end
.error:
ld a, 0x01
.end:
pop bc
pop de
pop hl
ret
; Read block index specified in DE and place the contents in buffer pointed to
; by (SDC_BUFPTR).
; If the operation is a success, updates buffer's sector to the value of DE.
; After a block read, check that CRC given by the card matches the content. If
; it doesn't, retries up to SDC_MAXTRIES times.
; Returns 0 in A if success, non-zero if error.
sdcReadBlk:
xor a
ld (SDC_RETRYCNT), a
push bc
push hl
out (SDC_PORT_CSLOW), a
.retry:
ld hl, 0
; DE already has the correct value
ld a, 0b01010001 ; CMD17
call sdcCmd
or a ; cp 0
jr nz, .error
; Command sent, no error, now let's wait for our data response.
ld b, 20
.loop1:
call sdcWaitResp
; 0xfe is the expected data token for CMD17
cp 0xfe
jr z, .loop1end
cp 0xff
jr nz, .error
djnz .loop1
jr .error ; timeout. error out
.loop1end:
; We received our data token!
; Data packets follow immediately, we have 512+CRC of them to read
ld bc, SDC_BLKSIZE+2
ld hl, (SDC_BUFPTR) ; HL --> active buffer's sector
; It sounds a bit wrong to set bufsec and dirty flag before we get our
; actual data, but at this point, we don't have any error conditions
; left, success is guaranteed. To avoid needlesssly INCing hl, let's
; set sector and dirty along the way
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ld (hl), e ; sector number LSB
inc hl
ld (hl), d ; sector number MSB
inc hl ; dirty flag
xor a ; unset
ld (hl), a
inc hl ; actual contents
.loop2:
call sdcIdle
ld (hl), a
cpi ; a trick to inc HL and dec BC at the same time.
; P/V indicates whether BC reached 0
jp pe, .loop2 ; BC is not zero, loop
; Success! while the card is busy, let's get busy too: let's check and
; see if the CRC matches.
push de ; <|
call sdcCRC ; |
; before we check the CRC r|esults, let's wait until card is ready
call sdcWaitReady ; |
; check CRC results |
ld a, (hl) ; |
cp d ; |
jr nz, .crcMismatch ; |
inc hl ; |
ld a, (hl) ; |
cp e ; |
jr nz, .crcMismatch ; |
pop de ; <|
; Everything is fine and dandy!
xor a ; success
jr .end
.crcMismatch:
; CRC of the buffer's content doesn't match the CRC reported by the
; card. Let's retry.
pop de ; from the push right before call sdcCRC
ld a, (SDC_RETRYCNT)
inc a
ld (SDC_RETRYCNT), a
cp SDC_MAXTRIES
jr nz, .retry ; we jump inside our main stack push. No need
; to pop it.
; Continue to error condition. Even if we went through many retries,
; our stack is still the same as it was at the first call. We can return
; normally (but in error condition).
.error:
; try to preserve error code
or a ; cp 0
jr nz, .end ; already non-zero
inc a ; zero, adjust
.end:
out (SDC_PORT_CSHIGH), a
pop hl
pop bc
ret
; Write the contents of buffer where (SDC_BUFPTR) points to in sector associated
; to it. Unsets the the buffer's dirty flag on success.
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; Before writing the block, update the buffer's CRC field so that the correct
; CRC is sent.
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; A returns 0 in A on success (with Z set), non-zero (with Z unset) on error.
sdcWriteBlk:
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push ix
ld ix, (SDC_BUFPTR) ; HL points to sector LSB
xor a
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cp (ix+2) ; dirty flag
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pop ix
ret z ; don't write if dirty flag is zero
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push bc
push de
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push hl
call sdcCRC ; DE -> new CRC. HL -> pointer to buf CRC
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ld (hl), d ; write computed CRC
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inc hl
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ld (hl), e
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out (SDC_PORT_CSLOW), a
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ld hl, (SDC_BUFPTR) ; sector LSB
ld e, (hl) ; sector LSB
inc hl
ld d, (hl) ; sector MSB
ld hl, 0 ; high addr word always zero, DE already set
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ld a, 0b01011000 ; CMD24
call sdcCmd
or a ; cp 0
jr nz, .error
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; Before sending the data packet, we need to send at least one empty
; byte.
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call sdcIdle
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; data packet token for CMD24
ld a, 0xfe
call sdcSendRecv
; Sending our data token!
ld bc, SDC_BLKSIZE+2 ; +2 for CRC. (as of now, however, that
; CRC isn't properly updated. Because
; CMD59 isn't enabled, it doesn't
; matter)
ld hl, (SDC_BUFPTR)
inc hl ; sector MSB
inc hl ; dirty flag
inc hl ; beginning of contents
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.loop:
ld a, (hl)
call sdcSendRecv
cpi ; a trick to inc HL and dec BC at the same time.
; P/V indicates whether BC reached 0
jp pe, .loop ; BC is not zero, loop
; Let's see what response we have
call sdcWaitResp
and 0b00011111 ; We ignore the first 3 bits of the response.
cp 0b00000101 ; A valid response is "010" in bits 3:1 flanked
; by 0 on its left and 1 on its right.
jr nz, .error
; good! Now, we need to let the card process this data. It will return
; 0xff when it's not busy any more.
call sdcWaitResp
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; Success! Now let's unset the dirty flag
ld hl, (SDC_BUFPTR)
inc hl ; sector MSB
inc hl ; dirty flag
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xor a
ld (hl), a
; Before returning, wait until card is ready
call sdcWaitReady
xor a
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jr .end
.error:
; try to preserve error code
or a ; cp 0
jr nz, .end ; already non-zero
inc a ; zero, adjust
.end:
out (SDC_PORT_CSHIGH), a
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pop hl
pop de
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pop bc
ret
; 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 SD card. If the selected
; buffer doesn't have the same sector as what EHL asks, load that buffer from
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; the SD card.
; If the dirty flag is set, we write the content of the in-memory buffer to the
; 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
; sdcReadBlk or sdcWriteBlk)
sdcSync:
push de
; Given a 24-bit address in EHL, extracts the 15-bit sector from it and
; place it in DE.
; We need to shift both E and H right by one bit
srl e ; sets Carry
ld d, e
ld a, h
rra ; takes Carry
ld e, a
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; Let's first see if our first buffer has our sector
ld a, (SDC_BUFSEC1) ; sector LSB
cp e
jr nz, .notBuf1
ld a, (SDC_BUFSEC1+1) ; sector MSB
cp d
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jr z, .buf1Ok
.notBuf1:
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; Ok, let's check for buf2 then
ld a, (SDC_BUFSEC2) ; sector LSB
cp e
jr nz, .notBuf2
ld a, (SDC_BUFSEC2+1) ; sector MSB
cp d
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jr z, .buf2Ok
.notBuf2:
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; None of our two buffers have the sector we need, we'll need to load
; a new one.
; We select our buffer depending on which is dirty. If both are on the
; same status of dirtiness, we pick any (the first in our case). If one
; of them is dirty, we pick the clean one.
push de ; <|
ld de, SDC_BUFSEC1 ; |
ld a, (SDC_BUFDIRTY1) ; |
or a ; | is buf1 dirty?
jr z, .ready ; | no? good, that's our buffer
; yes? then buf2 is our buffer. ; |
ld de, SDC_BUFSEC2 ; |
; |
.ready: ; |
; At this point, DE points to one o|f our two buffers, the good one.
; Let's save it to SDC_BUFPTR |
ld (SDC_BUFPTR), de ; |
; |
pop de ; <|
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; We have to read a new sector, but first, let's write the current one
; if needed.
call sdcWriteBlk
jr nz, .end ; error
; Let's read our new sector in DE
call sdcReadBlk
jr .end
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.buf1Ok:
ld de, SDC_BUFSEC1
ld (SDC_BUFPTR), de
; Z already set
jr .end
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.buf2Ok:
ld de, SDC_BUFSEC2
ld (SDC_BUFPTR), de
; Z already set
; to .end
.end:
pop de
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ret
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; Computes the CRC-16, with polynomial 0x1021 of buffer at (SDC_BUFPTR) and
; returns its value in DE. Also, make HL point to the first byte of the CRC
; associated to (SDC_BUFPTR).
sdcCRC:
push af
push bc
ld hl, (SDC_BUFPTR)
inc hl \ inc hl \ inc hl ; HL points to contents
ld bc, SDC_BLKSIZE
ld de, 0
.loop:
push bc ; <|
ld b, 8 ; |
ld a, (hl) ; |
xor d ; |
ld d, a ; |
.inner: ; |
sla e ; | Sets Carry
rl d ; | Takes and sets carry
jr nc, .noCRC ; |
; msb was set, apply polyno|mial
ld a, d ; |
xor 0x10 ; |
ld d, a ; |
ld a, e ; |
xor 0x21 ; |
ld e, a ; |
.noCRC: ; |
djnz .inner ; |
pop bc ; <|
cpi ; inc HL, dec BC, sets P/V on BC=0
jp pe, .loop ; BC is not zero, loop
; At this point, HL points to the right place: the first byte of the
; recorded CRC.
pop bc
pop af
ret
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sdcInitializeCmd:
call sdcInitialize
ret nz
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call .setBlkSize
ret nz
call .enableCRC
ret nz
; At this point, our buffers are unnitialized. We could have some logic
; that determines whether a buffer is initialized in appropriate SDC
; routines and act appropriately, but why bother when we could, instead,
; just buffer the first two sectors of the card on initialization? This
; way, no need for special conditions.
; initialize variables
ld hl, SDC_BUFSEC1
ld (SDC_BUFPTR), hl
ld de, 0
call sdcReadBlk ; read sector 0 in buf1
ret nz
ld hl, SDC_BUFSEC2
ld (SDC_BUFPTR), hl
inc de
jp sdcReadBlk ; read sector 1 in buf2, returns
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; Send a command to set block size to SDC_BLKSIZE to the SD card.
; Returns zero in A if a success, non-zero otherwise
.setBlkSize:
push hl
push de
ld a, 0b01010000 ; CMD16
ld hl, 0
ld de, SDC_BLKSIZE
call sdcCmdR1
; Since we're out of idle mode, we expect a 0 response
; We need no further processing: A is already the correct value.
pop de
pop hl
ret
; Enable CRC checks through CMD59
.enableCRC:
push hl
push de
ld a, 0b01111011 ; CMD59
ld hl, 0
ld de, 1 ; 1 means CRC enabled
call sdcCmdR1
pop de
pop hl
ret
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; Flush the current SDC buffer if dirty
sdcFlushCmd:
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ld hl, SDC_BUFSEC1
ld (SDC_BUFPTR), hl
call sdcWriteBlk
ret nz
ld hl, SDC_BUFSEC2
ld (SDC_BUFPTR), hl
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jp sdcWriteBlk ; returns
; *** blkdev routines ***
; Make HL point to its proper place in SDC_BUF.
; EHL currently is a 24-bit offset to read in the SD card. E=high byte,
; HL=low word. Load the proper sector in memory and make HL point to the
; correct data in the memory buffer.
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_sdcPlaceBuf:
call sdcSync
ret nz ; error
; At this point, we have the proper buffer in place and synced in
; (SDC_BUFPTR). Only the 9 low bits of HL are important.
push de
ld de, (SDC_BUFPTR)
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inc de ; sector MSB
inc de ; dirty flag
inc de ; contents
ld a, h ; high byte
and 0x01 ; is first bit set?
jr z, .read ; first bit reset? we're in the "lowbuf" zone.
; DE already points to the right place.
; We're in the highbuf zone, let's inc DE by 0x100, which, as it turns
; out, is quite easy.
inc d
.read:
; DE is now placed either on the lower or higher half of the active
; buffer and all we need is to increase DE the lower half of HL.
ld a, l
call addDE
ex de, hl
pop de
; Now, HL points exactly at the right byte in the active buffer.
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xor a ; ensure Z
ret
sdcGetB:
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push hl
call _sdcPlaceBuf
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jr nz, .end ; NZ already set
; This is it!
ld a, (hl)
cp a ; ensure Z
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.end:
pop hl
ret
sdcPutB:
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push hl
push af ; let's remember the char we put, _sdcPlaceBuf
; destroys A.
call _sdcPlaceBuf
jr nz, .error
; HL points to our dest. Recall A and write
pop af
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, (SDC_BUFPTR)
inc hl ; sector MSB
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inc hl ; point to dirty flag
ld (hl), a ; set dirty flag
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xor a ; ensure Z
jr .end
.error:
; preserve error code
ex af, af'
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pop af
ex af, af'
call unsetZ
.end:
pop hl
ret