microdrive: fix a few bugs

With this done we can now do all the core bits of loading an image

diff --git a/Kernel/platform-zx128/microdrive.s b/Kernel/platform-zx128/microdrive.s
index 9b9b8ee..519c967 100644 (file)
--- a/Kernel/platform-zx128/microdrive.s
+++ b/Kernel/platform-zx128/microdrive.s
@@ -28,17 +28,12 @@
                .globl _mdv_bwrite
                .globl mdv_boot
 
-               .globl HBP
-
                ; imports
                .globl _mdv_sector
                .globl _mdv_buf
                .globl _mdv_hdr_buf
                .globl _mdv_len
 
-               .globl mdv_sync
-               .globl mdv_get_blk
-
 SECTORID       .equ    0x08            ; FIXME - set real format up!
 CSUM           .equ    0x0E            ; FIXME ditto
 
@@ -190,28 +185,36 @@ hdr_fail:
 ;      give up after 3 loops of the tape exactly.
 ;
 mdv_find_hdr:  ld bc, #2048            ; worst case is 4 times round the tape
-               push bc
-               call mdv_get_hdr
+
+mdv_find_hdr_l:        push bc
+               call mdv_get_hdr        ; Fetch any header
+               pop bc
+               jr nz, mdv_find_hdr_bad ; If it didn't work check the error
                ld hl, #_mdv_hdr_buf
-               ld a, (hl)
+               ld a, (hl)              ; Was it data ?
                cp #1
-               ret nz                  ; NZ, 1 = not a header
-               jr nz, mdv_find_hdr_bad
+               jr nz, mdv_find_hdr_next; NZ, 1 = not a header
                ld hl, #_mdv_hdr_buf + SECTORID
                ld a, (_mdv_sector)
                cp (hl)
-               pop bc
                ret z                   ; found it
-mdv_find_hdr_2:
+               ; Sector header, valid, but not the one we wanted
+mdv_find_hdr_next:
+               ; Count down through our tape scan
                dec bc
                ld a, b
                or c
-               jr nz, mdv_find_hdr     ; keep looking
+               jr nz, mdv_find_hdr_l   ; keep looking
                inc a                   ; NZ
                ret
+               ;
+               ; Error 3 from mdv_get_hdr means there was nothing found
+               ; on the tape, so no point trying further. Otherwise it was
+               ; just a bad header, and we can carry on
+               ;
 mdv_find_hdr_bad:
                cp #3                   ; 3 = give up now
-               jr nz, mdv_find_hdr_2
+               jr nz, mdv_find_hdr_next
                or a                    ; will be > 0
                ret                     ; NZ
 
@@ -365,9 +368,6 @@ _mdv_bwrite:
 ;      causes a stack overwrite, that's operator error!
 
 mdv_boot:
-
-               ld (0xfffe), sp
-               ld sp, #0xfffe
 ;
 ;      Spin up the boot volume
 ;
@@ -386,7 +386,6 @@ mdv_boot_loop:
 ;      the next block ok
 ;
                call mdv_get_hdr
-HBP:
                jr nz, mdv_bad
                call mdv_get_blk
                jr nz, mdv_bad
@@ -417,6 +416,8 @@ HBP:
                ldir
                call done_all           ; check if we are complete
                jr z, mdv_boot_done
+               ld hl, #0x4000          ; we may have reloaded over this
+               ld (_mdv_buf), hl
 not_fk:                pop hl
                dec hl
                ld a, h
@@ -433,7 +434,6 @@ mdv_bad:    cp #3
 mdv_boot_done:
                ld a, #0x7
                out (0xFE), a
-               ld sp, (0xfffe)
                ret
 
 done_all:      ld hl, #0x585B          ; check data is loaded
@@ -444,7 +444,7 @@ done_1:             cp (hl)
                inc hl
                djnz done_1
                ld hl, #0x58C0          ; and code
-               ld b, #0x3E             ; but not FE-FFFF
+               ld b, #0x3f
 done_2:                cp (hl)
                ret nz
                inc hl