collapseos/kernel/core.asm
Virgil Dupras 7274dccbe7 Move ASCII consts to ascii.h
And made them shorter in name. The new ascii.h file allow reuse in userspace
code.
2019-11-13 20:38:06 -05:00

243 lines
5.1 KiB
NASM

; core
;
; Routines used by pretty much all parts. You will want to include it first
; in your glue file.
; *** REGISTER FIDDLING ***
; add the value of A into DE
addDE:
push af
add a, e
jr nc, .end ; no carry? skip inc
inc d
.end:
ld e, a
pop af
noop: ; piggy backing on the first "ret" we have
ret
; copy (HL) into DE, then exchange the two, utilising the optimised HL instructions.
; ld must be done little endian, so least significant byte first.
intoHL:
push de
ld e, (hl)
inc hl
ld d, (hl)
ex de, hl
pop de
ret
intoDE:
ex de, hl
call intoHL
ex de, hl ; de preserved by intoHL, so no push/pop needed
ret
intoIX:
push ix
ex (sp), hl ;swap hl with ix, on the stack
call intoHL
ex (sp), hl ;restore hl from stack
pop ix
ret
; add the value of A into HL
; affects carry flag according to the 16-bit addition, Z, S and P untouched.
addHL:
push de
ld d, 0
ld e, a
add hl, de
pop de
ret
; subtract the value of A from HL
; affects flags according to the 16-bit subtraction.
subHL:
push de
ld d, 0
ld e, a
or a ;reset carry flag
sbc hl, de ;There is no 'sub hl, de', so we must use sbc
pop de
ret
; Compare HL with DE and sets Z and C in the same way as a regular cp X where
; HL is A and DE is X.
cpHLDE:
push hl
or a ;reset carry flag
sbc hl, de ;There is no 'sub hl, de', so we must use sbc
pop hl
ret
; Write the contents of HL in (DE)
; de and hl are preserved, so no pushing/popping necessary
writeHLinDE:
ex de, hl
ld (hl), e
inc hl
ld (hl), d
dec hl
ex de, hl
ret
; Call the method (IX) is a pointer to. In other words, call intoIX before
; callIX
callIXI:
push ix
call intoIX
call callIX
pop ix
ret
; jump to the location pointed to by IX. This allows us to call IX instead of
; just jumping it. We use IX because we seldom use this for arguments.
callIX:
jp (ix)
callIY:
jp (iy)
; Ensures that Z is unset (more complicated than it sounds...)
; There are often better inline alternatives, either replacing rets with
; appropriate jmps, or if an 8 bit register is known to not be 0, an inc
; then a dec. If a is nonzero, 'or a' is optimal.
unsetZ:
or a ;if a nonzero, Z reset
ret nz
cp 1 ;if a is zero, Z reset
ret
; *** STRINGS ***
; Fill B bytes at (HL) with A
fill:
push bc
push hl
.loop:
ld (hl), a
inc hl
djnz .loop
pop hl
pop bc
ret
; Increase HL until the memory address it points to is equal to A for a maximum
; of 0xff bytes. Returns the new HL value as well as the number of bytes
; iterated in A.
; If a null char is encountered before we find A, processing is stopped in the
; same way as if we found our char (so, we look for A *or* 0)
; Set Z if the character is found. Unsets it if not
findchar:
push bc
ld c, a ; let's use C as our cp target
ld a, 0xff
ld b, a
.loop: ld a, (hl)
cp c
jr z, .match
or a ; cp 0
jr z, .nomatch
inc hl
djnz .loop
.nomatch:
call unsetZ
jr .end
.match:
; We ran 0xff-B loops. That's the result that goes in A.
ld a, 0xff
sub b
cp a ; ensure Z
.end:
pop bc
ret
; Format the lower nibble of A into a hex char and stores the result in A.
fmtHex:
; The idea here is that there's 7 characters between '9' and 'A'
; in the ASCII table, and so we add 7 if the digit is >9.
; daa is designed for using Binary Coded Decimal format, where each
; nibble represents a single base 10 digit. If a nibble has a value >9,
; it adds 6 to that nibble, carrying to the next nibble and bringing the
; value back between 0-9. This gives us 6 of that 7 we needed to add, so
; then we just condtionally set the carry and add that carry, along with
; a number that maps 0 to '0'. We also need the upper nibble to be a
; set value, and have the N, C and H flags clear.
or 0xf0
daa ; now a =0x50 + the original value + 0x06 if >= 0xfa
add a, 0xa0 ; cause a carry for the values that were >=0x0a
adc a, 0x40
ret
; Formats value in A into a string hex pair. Stores it in the memory location
; that HL points to. Does *not* add a null char at the end.
fmtHexPair:
push af
; let's start with the rightmost char
inc hl
call fmtHex
ld (hl), a
; and now with the leftmost
dec hl
pop af
push af
rra \ rra \ rra \ rra
call fmtHex
ld (hl), a
pop af
ret
; Compares strings pointed to by HL and DE up to A count of characters. If
; equal, Z is set. If not equal, Z is reset.
strncmp:
push bc
push hl
push de
ld b, a
.loop:
ld a, (de)
cp (hl)
jr nz, .end ; not equal? break early. NZ is carried out
; to the called
cp 0 ; If our chars are null, stop the cmp
jr z, .end ; The positive result will be carried to the
; caller
inc hl
inc de
djnz .loop
; We went through all chars with success, but our current Z flag is
; unset because of the cp 0. Let's do a dummy CP to set the Z flag.
cp a
.end:
pop de
pop hl
pop bc
; Because we don't call anything else than CP that modify the Z flag,
; our Z value will be that of the last cp (reset if we broke the loop
; early, set otherwise)
ret
; Transforms the character in A, if it's in the a-z range, into its upcase
; version.
upcase:
cp 'a'
ret c ; A < 'a'. nothing to do
cp 'z'+1
ret nc ; A >= 'z'+1. nothing to do
; 'a' - 'A' == 0x20
sub 0x20
ret