83 lines
1.7 KiB
ArmAsm
83 lines
1.7 KiB
ArmAsm
// #LAYOUT# STD * #TAKE
|
|
// #LAYOUT# * BASIC_0 #TAKE
|
|
// #LAYOUT# * * #IGNORE
|
|
|
|
// Math package - fetch FAC1 from RAM location
|
|
//
|
|
// Input:
|
|
// - .A - address low byte
|
|
// - .Y - address high byte
|
|
//
|
|
// Output:
|
|
// - .A - FAC1 exponent, affects status flags
|
|
// - .Y - always 0
|
|
//
|
|
// Preserves:
|
|
// - .X - (experimentation with original ROM)
|
|
//
|
|
// See also:
|
|
// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
|
|
// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
|
|
|
|
// XXX test this
|
|
|
|
MOVFM:
|
|
|
|
// Checked in the available documentation:
|
|
// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
|
|
// - https://www.lemon64.com/forum/viewtopic.php?t=67576&sid=d41e77cdddd28d6f9c9d52ce5b4e8dc3
|
|
// that $22/$23 (INDEX+0 and INDEX+1) are at least sometimes used for pointer storage.
|
|
// Confirmed by experimenting with original ROM that this is the case here too.
|
|
|
|
sty INDEX+1
|
|
sta INDEX+0
|
|
|
|
// Now copy the data from RAM to FAC2, for the format description see:
|
|
// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic#Representation_in_the_C-64
|
|
|
|
// Copy the mantissa
|
|
|
|
ldy #$04
|
|
lda (INDEX), y
|
|
sta FAC1_mantissa+3
|
|
|
|
dey
|
|
lda (INDEX), y
|
|
sta FAC1_mantissa+2
|
|
|
|
dey
|
|
lda (INDEX), y
|
|
sta FAC1_mantissa+1
|
|
|
|
dey
|
|
lda (INDEX), y
|
|
ora #$80 // clear the sign bit
|
|
sta FAC1_mantissa+0
|
|
|
|
// Copy the sign
|
|
|
|
lda (INDEX), y
|
|
bpl !+ // assumption: non-negative numbers are more frequent
|
|
lda #$FF
|
|
skip_2_bytes_trash_nvz
|
|
!:
|
|
lda $00
|
|
|
|
sta FAC1_sign
|
|
|
|
// Copy the exponent
|
|
|
|
dey
|
|
lda (INDEX), y
|
|
sta FAC1_exponent
|
|
|
|
// Set low order mantissa to 0 - this should give the
|
|
// most precision while operating on integers
|
|
|
|
sty FACOV
|
|
|
|
// Return the FAC1 exponent
|
|
|
|
lda FAC1_exponent
|
|
rts
|