; ***************************************************************************
; Calculates the actual screen address for a character in BC (row,col)
; returning that address in HL.
;
; Author: Peter Mount, Area51.dev & Contributors
; URL: https://area51.dev/sinclair/asm/screen/getcharaddr/
; ***************************************************************************
;
; Although the memory layout for the spectrum screen seems weird,
; it's actually pretty logical. You can tell this weirdness is down to how
; the ULA works internally with the way the addressing is mapped, as shown
; at https://area51.dev/sinclair/spectrum/screen/file/
;
; Address format:
;           H       L
;       010rrnnn rrrccccc    where r=row,
;                                  c=column,
;                                  n=row in the character definition.
;
; To calculate the address of a character:
;       High byte = &40 or (row and &18)
;       Low byte  = (row<<5) or column
;
; That would then be the top byte of the character.
;
; To get the next byte in the character just increment the high byte
; (e.g. n=1) & so on for the next 7 bytes.
;
; To get the next character's address then simply increment the low byte.
; Be careful however when wrapping around the end of the line as, after
; the first 8 lines the address will fail.
;
; Ideally you should calculate the address again at the start of each line.
;
; This might explain why the original spectrum editor was 8 lines long, as you
; only needed to set the high byte to the 8-line block in memory and set the
; low byte to the offset in the line being edited to get that characters
; screen address.
;
; On Entry:
;   BC      b=row, c=col in Spectrum 32x24 characters
;
; On Exit:
;   HL      address of top row if character definition in screen
;   BC      unchanged
;   A       undefined
;
getSpectrumCharPos:             ; Get Spectrum char pos (bc) into HL for physical screen address
    ld      a, b                ; first calculate high byte - get row
    and     &18                 ; Mask bits 4 & 5 from row
    or      &40                 ; set bit 6, this gives us the upper memory address
    ld      h, a                ; Set H

    ld      a, b                ; get row
    add     a                   ; Shift left 5 to form low address
    add     a                   ; use add not srl as this saves 1 byte and half the t-states
    add     a                   ; per shift, especially as we don't need to worry about carry flag
    add     a
    add     a                   ; HA is now the address of the start of line

    or      c                   ; Add column value
    ld      l, a                ; HL now address of top row of character

    ret