source code for JSW

Norman Sword · July 17, 2019

Yes the BLOCKX_MOVE32 will clear the Parity Even Flag (PE flag) if and only if the last LDI counts the register pair "BC" down to zero.

Which is why the line that loads BC with 128 is specifically indicating that "BC" must be set to a multiple of 32. If this was changed to a value which is not divisible by 32. Then the attributes would never be drawn, because the ( "JP PE,n_raster" ) would always branch to ( "n_raster" )

Edited July 17, 2019 by Norman Sword

IRF · July 17, 2019

N.B. My method may complicate things in cases where a chunk of code is being overwritten with a single value - where the first byte is overwritten directly and then the number of bytes to which the same values is to be copied in a loop is minus one. e.g. for attribute update with a single value (such as for a screen flash effect), use #01FF instead of #0200 to define the size of the loop.

Norman's code deals with such cases by CALLing a late entry point into his subroutine, coinciding with the second LDI command in the subroutine. (But the JR NZ at the end of the subroutine jumps back to the first LDI in the subroutine.)

In such cases, I think my method would unavoidably end up 'overshooting', and overwriting one more byte than it should. (But in the aforementioned project, I didn't actually use an LDI method for 'block fill' purposes, only for 'block move'.)

On reflection, if I did want to use the same subroutine in the context of overwriting a block of code with a single value (e.g. to implement the Screen Flash effect), then it could be done like this (at the cost of five more bytes):

; 16 character rows to colour, so use A=#0F (15 in decimal) for the later loop.

LD HL, xxxx

LD DE. xxxx +1

; No need to define BC; it's not used now

LD (HL), colour_value ; Either copied from A, or a fixed value specified here

LD A, #01

CALL subroutine_late_entry

LD A, #0F

loop:

CALL subroutine

DEC A

JR NZ, loop

subroutine:

LDI

subroutine_late_entry:

rept 31 ; I presume "rept y" means 'repeat the following code (up to the end marker) y times'?

LDI

endm ; I presume this means 'end marker'?

RET

Edited July 17, 2019 by IRF

Norman Sword · July 17, 2019

Standard assembler directives:-

One of the biggest helps is the macro language that you can build into pieces of standard code that are often used.

The standard macro is as such:

silly: Macro param1,param2,param3

ld hl,param1

ld de,param2

ld bc,param3

ldir

endm.

The macro here is defined by the label silly. Every time the word silly is seen in the source code it will replace it with all the code between the MACRO and endm.

The word endm is shorthand for "end macro"

So How do I use silly in a piece of code

( don't forget this example is called silly, because it is a silly example)

I simply write in to the assemble code

silly ATT0,ATT1,32

when the code is assembled the above will be substituted with

ld hl,ATT0

ld de,ATT1

ld bc,32

LDIR

which is the macro expanded and each of the labels re placed with the parameter passed to the macro.
e.g
ld hl,ATT0 ; here param1 is replaced with ATT0
       ld de,ATT1 ; here param2 is replaced with ATT1
       ld bc,32 ; here param3 is replaced with 32
       LDIR

Macros are very helpful to expand code out that is repetitive.

another form of macros. is the rept directive. (rept = repeat). so getting back to to the original querry

rept 32 ;repeat 32 times

ldi

endm ; end macro

means repeat the line of code between the rept directive and then endm directive 32 times

we can create big blocks of code by repeating and nesting : (these examples are just examples not taken from any code)

so lets look at

move macro count

rept count

ld a,(hl)

ld (de),a

inc hl

inc d

endm

we can in the assembler now write

move 8

and this will generate the inline code

ld a,(hl)
                  ld (de),a
                  inc hl
                  inc d

ld a,(hl)