fs: hard links - ln(1), and rm that removes a NAME

gbfs has carried I_NLINK since the beginning and nothing ever incremented it,
so a file could only have one name. SYS_LINK adds a second directory entry
pointing at the same inode: the data is stored once, and both names read it.

That makes rm's job different. It now decrements the link count and only frees
the inode and its blocks when the LAST name goes - removing one of two links
used to free blocks the other still pointed at.

Refused, with the reasons that matter here: linking a directory (it would make
a cycle the tree walkers cannot survive), an existing name, and anything under
/bin, which is ROM.

The syscall takes a request block because the trap needs HL for its dispatch
table - and ln copies both paths to the stack first, since that block is a
static living at $A000, on top of the command line it is reading.

1 files changed, 120 insertions, 0 deletions

diff --git a/src/syscall.asm b/src/syscall.asm
index b548328..a4719fc 100644
--- a/src/syscall.asm
+++ b/src/syscall.asm
@@ -74,6 +74,9 @@ SyscallTable:
     dw sys_stat                 ; 37 STAT
     dw sys_fsstat               ; 38 FSSTAT
     dw sys_isatty               ; 39 ISATTY
+    dw sys_link                 ; 40 LINK
+    dw sys_settime              ; 41 SETTIME
+    dw sys_gettime              ; 42 GETTIME
 
 ; -----------------------------------------------------------------------------
 sys_nosys:
@@ -335,6 +338,123 @@ sys_sleep:
     ret
 
 ; -----------------------------------------------------------------------------
+; sys_settime(DE = buf[4]) - unix seconds, little-endian. There is no RTC on an
+; MBC5 cart, so the wall clock is an epoch stamp plus the tick it was taken at
+; (ntpdate(1) fetches the stamp over SNTP); sys_gettime adds the elapsed ticks
+; back. It does not survive a power cycle - run ntpdate from /rc.
+; -----------------------------------------------------------------------------
+sys_settime:
+    ld hl, wEpoch
+    ld b, 4
+.cp
+    ld a, [de]
+    ld [hl+], a
+    inc de
+    dec b
+    jr nz, .cp
+    ld hl, wTicks               ; remember when this stamp was true
+    ld de, wEpochTick
+    ld b, 4
+.cpt
+    ld a, [hl+]
+    ld [de], a
+    inc de
+    dec b
+    jr nz, .cpt
+    xor a
+    ret
+
+; -----------------------------------------------------------------------------
+; sys_gettime(DE = buf[4]) -> A = 1 if the clock has been set, 0 if not.
+; buf = wEpoch + (wTicks - wEpochTick) / 64, little-endian.
+; -----------------------------------------------------------------------------
+sys_gettime:
+    push de
+    ; elapsed = wTicks - wEpochTick, byte-wise with borrow (32-bit)
+    ld hl, wTimeTmp
+    ld a, [wTicks+0]
+    ld b, a
+    ld a, [wEpochTick+0]
+    ld c, a
+    ld a, b
+    sub c
+    ld [hl+], a
+    ld a, [wTicks+1]
+    ld b, a
+    ld a, [wEpochTick+1]
+    ld c, a
+    ld a, b
+    sbc c
+    ld [hl+], a
+    ld a, [wTicks+2]
+    ld b, a
+    ld a, [wEpochTick+2]
+    ld c, a
+    ld a, b
+    sbc c
+    ld [hl+], a
+    ld a, [wTicks+3]
+    ld b, a
+    ld a, [wEpochTick+3]
+    ld c, a
+    ld a, b
+    sbc c
+    ld [hl], a
+    ; elapsed >>= 6  (ticks -> seconds), 32-bit, six single-bit shifts
+    ld b, 6
+.shift
+    ld hl, wTimeTmp+3
+    srl [hl]
+    dec hl
+    rr [hl]
+    dec hl
+    rr [hl]
+    dec hl
+    rr [hl]
+    dec b
+    jr nz, .shift
+    ; out = wEpoch + elapsed
+    pop de
+    ld hl, wEpoch
+    ld a, [hl+]
+    ld b, a
+    ld a, [wTimeTmp+0]
+    add b
+    ld [de], a
+    inc de
+    ld a, [hl+]
+    ld b, a
+    ld a, [wTimeTmp+1]
+    adc b
+    ld [de], a
+    inc de
+    ld a, [hl+]
+    ld b, a
+    ld a, [wTimeTmp+2]
+    adc b
+    ld [de], a
+    inc de
+    ld a, [hl]
+    ld b, a
+    ld a, [wTimeTmp+3]
+    adc b
+    ld [de], a
+    ; A = is the clock set? (wEpoch != 0)
+    ld a, [wEpoch+0]
+    ld b, a
+    ld a, [wEpoch+1]
+    or b
+    ld b, a
+    ld a, [wEpoch+2]
+    or b
+    ld b, a
+    ld a, [wEpoch+3]
+    or b
+    ret z
+    ld a, 1
+    ret
+
+; -----------------------------------------------------------------------------
 ; sys_isatty() -> A = 1 if this process's stdin is the console, 0 if it is a
 ; file or a pipe. The shell uses it to tell an interactive session from a
 ; script (`sh < file`): no prompt, no echo, and EOF is just the end.