kissa.bsky.social
@kissa.bsky.social
Tomfoolery, nincompoopery, programming
37 followers374 following39 posts
Operands are now simply 8 bits and denote memory addresses 0x00 to 0xff. I have a slightly more complex design in mind but let's get some syntax defined first. This also details how to address lower and higher parts of registers:
![Syntax
[M] means the word at memory address M.
[M].b means the byte at memory address M. When read, it is sign-extended
to a word.
<reg> is equivalent to [addressof <reg>], where <reg> is one of registers
(a, b, c, d, x, y, i, j, p, q, sp, pc), and addressof <reg> is the memory
address of the register according to the table below.
<reg>.hi is equivalent to [address of <reg>].b
<reg>.lo is equivalent to [address of <reg> + 1].b
Table of registers and their addresses:
a: 0x0 a.hi: 0x0 a.lo: 0x1
b: 0x2 b.hi: 0x2 b.lo: 0x3
c: 0x4 c.hi: 0x4 c.lo: 0x5
d: 0x6 d.hi: 0x6 d.lo: 0x7
x: 0x8 x.hi: 0x8 x.lo: 0x9
y: 0xa y.hi: 0xa y.lo: 0xb
i: 0xc i.hi: 0xc i.lo: 0xd
j: 0xe j.hi: 0xe j.lo: 0xf
p: 0x10 p.hi: 0x10 p.lo: 0x11
q: 0x12 q.hi: 0x12 q.lo: 0x13
sp: 0x14 sp.hi: 0x14 sp.lo: 0x15
pc: 0x16 pc.hi: 0x16 pc.lo: 0x17
Finally:
<regi> is one of x, y, i, j
<rega> is one of p, q, sp, pc](https://cdn.bsky.app/img/feed_fullsize/plain/did:plc:ih3rt6r3sesh2flndp7oefwc/bafkreihgoilh3oo5wjtu76jpvjf6cyjdj42fqfy7bg6gxysyg7nwxhkcqi@jpeg)
What we have now is direct addressing, e.g. [0x1234]. This extends the design with indirect addressing using either immediate or an index register, allowing things like [pc + 5] and [p + i]. Also byte addressing is supported with syntax [0x1234].b
![Operand format:
Direct addressing (word), values 0..63:
00mmmmmm: [M], where M is number 0-63
Direct addressing (byte), values 64..127:
01mmmmmm: [M].b, where M is number 0-63
Indirect addressing with immediate offset (word), values 128..175:
10ooooaa: [<rega> + O], where <rega> is selected with bits 'aa' and O is
the range 0b0000-0b1011, represented by bits 'oooo', mapped to values -6 to 5
Indirect addressing with index register (word), values 176..191:
1011iiaa: [<rega> + <regi>], where <rega> is selected with bits 'aa' and
<regi> is selected with bits 'ii'
Indirect addressing with immediate offset (byte), values 192..239:
11ooooaa: [<rega> + O].b, where <rega> is selected with bits 'aa' and O is
range 0b0000-0b1011 mapped to values -6 to 5
Indirect addressing with index register (byte), values 240..255:
1111iiaa: [<rega> + <regi>].b, where <rega> is selected with bits 'aa' and
<regi> is selected with bits 'ii'](https://cdn.bsky.app/img/feed_fullsize/plain/did:plc:ih3rt6r3sesh2flndp7oefwc/bafkreih3zx7t3wq6w3pkopewejmdrb52v773w6aqxjls7f7degsewkj6ba@jpeg)
kissa.bsky.social
@kissa.bsky.social
Tomfoolery, nincompoopery, programming
37 followers374 following39 posts