VARCem/asm6502
1
0
Fork 0
mirror of https://github.com/VARCem/asm6502.git synced 2026-09-24 23:57:14 +00:00
Code Issues Packages Projects Releases Wiki Activity

work on documentation

Browse Source
This commit is contained in:
Bernd Boeckmann
2022-10-14 17:16:06 +02:00
parent 792fc3d58a
commit 90cede6fc3
1 changed files with 13 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files

26
README.md
View File

@@ -88,16 +88,16 @@ Each line may end with a comment. Comments start with a semicolon.
At the beginning of a line a label may be specified if the line does not contain a variable definition.
start: ; line consisting of a label comment
loop: BNE loop ; label, instruction and comment
msg: .byte "Hello" ; label followed by a directive and comment
start: ; line consisting of a label
loop: BNE loop ; label and instruction
msg: .byte "Hello" ; label followed by a directive
Variables are defined by giving the variable name followed by equal sign followed by an expression yielding a numeric value:
CHROUT = $FFD2
## Directives
Directive instruct the assembler to do certain things. They do not yield machine code but may produce output data. Names of directives start with a dot. The directives currently known to the assembler are:
Directive instruct the assembler to do certain things. They may or may not produce output data. Names of directives start with a dot. The directives currently known to the assembler are:
### .ORG directive
Sets the current program counter to the numeric value of the argument
@@ -158,7 +158,7 @@ The address is encoded in the word following the opcode and displaced by the con
### Zeropage X and zeropage Y addressing
The address is encoded in the byte following the opcode and displaced by the contents for the X or Y register.
LDA <$4711,X ; load contents of address $11 displaced by X
LDA $47,X ; load contents of address $47 displaced by X
LDX >$4711,Y ; load contents of address $47 displaced by Y into X
### Indirect addressing
@@ -166,7 +166,7 @@ The word sized address is stored in the memory location given by the word sized
JMP ($4711)
This one is a syntax error, because the assembler assumes indirect addressing mode instead of a subexpression grouped by parentheses:
The following one is a syntax error, because the assembler assumes indirect addressing mode instead of a subexpression grouped by parentheses:
JMP (2+3)*1000
@@ -174,11 +174,11 @@ If one wants to start an expression with ( while not indicating indirect address
JMP +(2+3)*1000
Examples are given below:
This one is correct (indirect addressing):
CLC ; clear carry: implicit addressing
ROR ; rotate right: accumulator addressing
LDA #42 ; load A with 42: immediate addressing mode
LDA $4711 ; load A with value at absolute address $4711
LDA $47 ; load A with value of zero page address $47
JMP ((2+3)*1000)
## Indirect X and indirect Y addressing
ORA (15,x)
ORA (15),y