Rewrite fputc in assembly

-36 bytes, -12% cycles

libsrc/common/fputc.s

@@ -0,0 +1,72 @@
+;
+; Colin Leroy-Mira, 2024
+;
+; int __fastcall__ fputc (int c, FILE* f);
+;
+
+        .export         _fputc
+        .importzp       ptr1
+        .import         _write
+        .import         pushax, pusha0, popax, incsp2
+        .import         pushptr1, popptr1, returnFFFF
+
+        .include        "stdio.inc"
+        .include        "_file.inc"
+
+_fputc:
+        sta     ptr1
+        stx     ptr1+1
+
+        jsr     popax           ; Get char, as we'll have
+        sta     c               ; to return it anyway
+        stx     c+1
+
+        ldy     #_FILE::f_flags
+        lda     (ptr1),y
+        tax
+        and     #_FOPEN         ; Check for file open
+        beq     ret_eof
+        txa
+        and     #(_FERROR|_FEOF); Check for error/eof
+        bne     ret_eof
+
+        jsr     pushptr1        ; Backup fp pointer
+
+        ; Push _write parameters
+        ldy     #_FILE::f_fd
+        lda     (ptr1),y
+        jsr     pusha0
+
+        lda     #<c
+        ldx     #>c
+        jsr     pushax
+
+        lda     #$01
+        ldx     #$00
+
+        ; Write
+        jsr     _write
+
+        ; Check for errors
+        cmp     #$01
+        bne     set_ferror
+
+        ; Return char
+        lda     c
+        ldx     #$00
+        jmp     incsp2          ; Drop fp pointer copy
+
+ret_eof:
+        jmp     returnFFFF
+
+set_ferror:
+        jsr     popptr1
+        lda     #_FERROR
+        ldy     #_FILE::f_flags
+        ora     (ptr1),y
+        sta     (ptr1),y
+        jmp     returnFFFF
+
+        .bss
+
+c:      .res 2