From 8b174acd0e91552d323d9d35ace4b930f68d2bd4 Mon Sep 17 00:00:00 2001 From: Clanmaster21 Date: Thu, 10 Oct 2019 21:07:23 +0100 Subject: [PATCH] addHL and subHL affect flags, and are smaller Most importantly, addHL and subHL now affect the flags as you would expect from a 16 bit addition/subtraction. This seems like it'd be preferred behaviour, however I realise any code relying on it not affecting flags would break. One byte saved in addHL, and two bytes saved in subHL. Due to the branching nature of the original code, it's difficult to compare speeds, subHL is either 1 or 6 cycles faster depending on branching, and addHL is between -1 and 3 cycles faster. If the chance of a carry is 50%, addHL is expected to be a cycle faster, but for a chance of carry below 25% (so a < 0x40) this will be up to a cycle slower. --- kernel/core.asm | 33 ++++++++++++++------------------- 1 file changed, 14 insertions(+), 19 deletions(-) diff --git a/kernel/core.asm b/kernel/core.asm index f23475a..b7a4af9 100644 --- a/kernel/core.asm +++ b/kernel/core.asm @@ -52,29 +52,24 @@ intoIX: pop ix ret -; add the value of A into HL +; add the value of A into HL, affecting flags like an ordinary addition addHL: - push af - add a, l - jr nc, .end ; no carry? skip inc - inc h -.end: - ld l, a - pop af + push de + ld d, 0 + ld e, a + add hl, de + pop de ret -; subtract the value of A from HL + +; subtract the value of A from HL, affecting flags like an ordinary subtraction subHL: - push af - ; To avoid having to swap L and A, we sub "backwards", that is, we add - ; a NEGated value. This means that the carry flag is inverted - neg - add a, l - jr c, .end ; if carry, no carry. :) - dec h -.end: - ld l, a - pop af + 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