CPU — KR580VM1

Soviet 8080 extension (KR580VM80A plus extras). Binary compatible with the 8080. Every 8080 program runs unchanged. It is not an 8085: the two chips reuse the same undocumented opcode slots for different instructions.

z88dk writes Zilog mnemonics. The Russian document uses Intel-style names (DSUB, LHLX, ANX). Translate on the way in.

Classic path libsrc/l/sccz80/7-vm1/, products vm1_crt0.lib / testvm1_clib.lib / vm1_clib.lib. Assemble -mvm1. zcc +test -clib=vm1 or +z80 -clib=vm1. Float: --math-mbf32 (mbf32_vm1.lib). No math32 / math16 vm1 product — those 8085 cores use ld de,sp+* and rl de, which are prefixes on this CPU.

Conventions (always follow)

1. Mnemonics are Zilog (ld a,b, sub hl,bc). Do not emit Intel MOV / DSUB / LHLX in tree sources (fixtures may list Intel for compat).
2. Opcode bytes and timings are 8080 + VM1 extras, not Z80 or 8085.
3. Immediate forms: * = d8, ** = d16/a16. Placeholders only.
4. Conditional cycle counts: document lists taken only. z88dk uses 8080 not-taken (jp always 10; call 17/11; ret 11/5).
5. Each prefix byte adds 4T. Base timings match the 8080.
6. One major function per file under libsrc.

Human page of record: doc/kr580vm1.md. When that page and an external Russian source disagree, the local page wins (it already folds the errata). Assembler last resort: cpu_test_vm1_ok.asm.

Sources

Topic	Source
ISA, prefixes, flags, timings, errata	doc/kr580vm1.md (primary)
Raw Russian ISA (empty JS wiki — fetch the storage URL)	radio86 KP580BM1.wiki
Secondary Russian	emuverse КР580ВМ1/Система_команд
z80asm source forms	src/z80asm/dev/cpu/cpu_test_vm1_ok.asm / _strict_ok.asm
sccz80 overrides	libsrc/l/sccz80/7-vm1/

Errata in the original Russian document

Resolved in doc/kr580vm1.md against emuverse and 8085 precedent. Use the local encodings.

Claim in the radio86 document	Truth
SHLX listed as 11001101 ($CD, 8080 CALL)	$D9, same as 8085 SHLX
SMF0/SMF1 tables show prefix $38 (RS)	Prefix is MB $28: smf0 = $28 $00, smf1 = $28 $7F

LHLX at $ED is listed correctly in the original.

Registers

8080 set plus one extra pointer pair. No IX/IY, no exx, no AF′/BC′/DE′.

15 ...... 8  7 ...... 0
     A            F
     B            C
     D            E
     H            L      → HL   (pointer pair)
     H'           L'     → HL'  (H1L1; z88dk `h'` `l'` `hl'`)
15 ............... 0
        SP
        PC

H1L1 is not a Z80 shadow bank. There is no exchange. The RS prefix selects H1L1 as source, destination, or (hl) pointer for one following instruction. ex de,hl always uses real HL (document gives no RS form).

z88dk ticks implements HL′ by parking it in the unused Z80 h_/l_ for the duration of one prefixed instruction.

Flag register (F)

8080 layout plus OF and MF in bits the 8080 left constant / copies of A:

Bit	7	6	5	4	3	2	1	0
	S	Z	OF	AC	MF	P	1	CY

Flag	Meaning
S	Sign
Z	Zero
OF	Signed overflow (carry into bit 7 xor carry out). Not 8085 K
AC	Half-carry
MF	Data bank: 0 = main, 1 = additional
P	Parity (even). Always parity, not Z80 P/V
1	Unused; permanently 1
CY	Carry

OF occupies the same bit as 8085 K. They are not the same flag. Do not use jp k / jp nk — those encodings are DCMP D / JOF on this CPU.

Condition codes: nz z nc c po pe p m, plus of (jp of,**).

Memory and MF

Two 64K banks. Instructions fetch only from the main bank. Data uses the bank named by MF. The MB prefix flips the bank for the next instruction only. smf0 / smf1 set MF until changed.

The document does not except the stack. z88dk therefore lets push / pop / call / ret follow MF. That is an inference. Changing MF between a call and its ret will not work.

Hard rule: pop af is not a free register

pop af loads MF from bit 3 of the popped byte. MF then selects the data bank for every later data access, including further pushes, pops, and ret.

Allowed	Forbidden
Matched push af … pop af (MF round-trips)	pop af as a stack discard
	push bc / pop af as a 16-bit move

Either forbidden form switches bank whenever bit 3 happens to be set — data-dependent, easy to miss. sccz80 and 80cc already suppress the idiom (CPU_POP_AF_IS_SAFE()). Hand-written asm must not invent it.

A pop af of a return address is also broken on 8080/8085/gbz80 (hardwired flag bits). On VM1 it is worse: it can switch banks as well. Keep return addresses in BC/DE/HL.

Prefixes

Up to two prefix bytes before any opcode. Opcode byte must not be $28 or $38. A run of prefixes does not cancel. Interrupts are held off between a prefix and its instruction (same idea as Rabbit altd).

RS — $38, 4T

Substitute HL′ for HL in the next instruction.

    ld  a,(hl')        ; 38 7E
    ld  a,h'
    ld  hl',nn
    push hl'
    add hl',bc

MB / CS — $28, 4T

MB in front of a memory op: access the other data bank.

    mb ld a,(hl)       ; 28 7E   A ← other bank at HL

CS in front of add hl,rp / sub hl,rp / cp hl,rp: consume carry. z88dk folds CS into the mnemonic (no separate mb/cs word in the source):

Bare	Prefixed	Meaning
add hl,bc (09)	adc hl,bc (28 09)	HL ← HL + BC + CY
sub hl,bc (08)	sbc hl,bc (28 08)	HL ← HL − BC − CY
cp hl,bc (CB)	cpc hl,bc (28 CB)	flags from HL − BC − CY

cpc is a z88dk name; the document has no Zilog spelling for compare-with-borrow.

Both prefixes, MB then RS:

    sbc hl',bc         ; 28 38 08

Added instructions

Timings are bare; add 4T per prefix.

Op	Zilog (write this)	Doc	T	Effect
08	sub hl,bc	DSUB B	10	HL ← HL − BC
18	sub hl,de	DSUB D	10	HL ← HL − DE
10	ld (hl),and (hl)	ANX	10	(HL) ← (HL) & A
20	ld (hl),or (hl)	ORX	10	(HL) ← (HL) | A
30	ld (hl),xor (hl)	XRX	10	(HL) ← (HL) ^ A
CB	cp hl,bc	DCMP B	10	flags from HL − BC
DD	cp hl,de	DCMP D	10	flags from HL − DE
D9	ld (de),hl	SHLX	10	(DE)←L, (DE+1)←H
ED	ld hl,(de)	LHLX	10	L←(DE), H←(DE+1)
FD **	jp of,**	JOF	10	jump if OF set
28 00	smf0	SMF0	8	MF ← 0
28 7F	smf1	SMF1	9	MF ← 1

These sit in 8080 unused cells ($x0/$x8 in the first quarter, and $CB/$D9/$DD/$ED/$FD). They are not Z80 prefixes at those bytes.

Flags on extras (from the local doc)

Instruction	S	Z	OF	AC	P	CY
DSUB	●	●				●
DCMP	●	●				●
DAD (add hl,rp)						●
ANX / ORX / XRX	●	●			●	0

DSUB S is bit 15 of the 16-bit result. Logical memory ops clear CY; OF and AC unchanged. DSUB/DCMP do not set OF (unlike a naïve “overflow bit” reading). Signed compares cannot use jp of after DSUB the way 8085 uses jp k after sub hl,bc.

Comparison with the 8085

Only DSUB B, SHLX, and LHLX match. Do not copy 8085 sequences that use the other slots.

Op	8085	KR580VM1
08	sub hl,bc	sub hl,bc — same
10	sra hl	ANX
18	rl de	sub hl,de
20	rim	ORX
28	ld de,hl+*	MB/CS prefix
30	sim	XRX
38	ld de,sp+*	RS prefix
CB	rst v	cp hl,bc
D9	ld (de),hl	same
DD	jp nk	cp hl,de
ED	ld hl,(de)	same
FD	jp k	jp of,**

There is no ld de,sp+* / ld de,hl+* / rl de / sra hl native. Frame addresses go through HL (ld hl,nn / add hl,sp). That is why vm1 trails 8085 on l_gintNsp and why math32/math16 8085 cores cannot be reused.

Coding rules for this project

1. Emit Zilog only.
2. Register pairs: bc, de, hl, hl', af, sp.
3. Prefer VM1 extras when they win: cp hl,de, sub hl,de, ld hl,(de), ld (de),hl.
4. Never emit 8085 encodings for LDSI / LDHI / RDEL / ARHL / RSTV / JNK / JK.
5. Never pop af to discard or to park a word.
6. Assembler last resort: src/z80asm/dev/cpu/cpu_test_vm1_{ok,err}.asm and *_strict_*. ok = accepted (native, synthetic, or call __z80asm__*). err = rejected. _strict_ = synthetics forbidden. Fixtures list Intel spellings; tree work is Zilog. tool-z80asm. rg one mnemonic. Do not bulk-read *_err.asm.

Ticks: z88dk-ticks -mvm1 binary — CPU flag before the path. Disassemble: z88dk-dis -mvm1.

$ED $FE remains the ticks hook: ld hl,(de) with $FE next is a hook, not LHLX.

No lib/arch/vm1/vm1_rules.1. Missing CPU rules file means no extra peepholes; the build is correct.

__CPU_INTEL__ is set (-D__8080 -D__VM1). 9-common IF __CPU_INTEL__ takes the 8080-portable path. VM1 extras live in 7-vm1/ and l/sccz80/vm1.lst.

Usage (from libsrc/l/sccz80/7-vm1/)

Cite the named file as the example. Every word extra reads or writes DE, so these helpers clobber DE. sccz80 does not keep DE live across them (checked against lib/z80rules.*).

1. 16-bit compare — cp hl,de (HL survives)

    cp  hl,de          ; l_eq.asm, l_ult.asm, …
    jp  z,equal

8085 needs ld bc,de / sub hl,bc because its DSUB only reaches BC. Here DCMP D is one byte and does not destroy HL.

2. Word load/store through DE — LHLX / SHLX

    ex  de,hl
    ld  hl,(de)        ; l_gint.asm / l_gintsp.asm

    ld  (de),hl        ; l_pint_pop.asm

l_gintsp: add hl,sp / inc hl×2 / ex de,hl / ld hl,(de) — no LDSI.

3. 16-bit subtract — sub hl,de

    ex  de,hl
    sub hl,de          ; l_sub.asm; CY is borrow (lib3d turn_left `ret nc`)

4. Second pointer — RS / hl', not exx

Keep a second 16-bit pointer in HL′ and prefix the ops that need it. Do not invent exx.

5. Other data bank — MB, then restore MF if the rest of the routine must stay on the original bank (smf0/smf1 or a matched push af/pop af of a known F).

What 7-vm1 deliberately does not override

Signed l_lt / l_le / l_gt / l_ge: DSUB/DCMP do not set OF, so there is no jp of shortcut. Sign-flip + DCMP is larger and slower than 9-common. l_asr / l_rlde / l_gint1sp… need 8085 ARHL/RDEL/LDSI — leave 8080 forms.

Synthetic opcodes (z80asm, normal mode)

Same word-copy family as other CPUs: ld among bc / de / hl (any → any) = two 8-bit loads. Prefer ld bc,hl … ld hl,bc to park a pair. Not af / sp.

Strict forbids free synthetics. Prefix forms (adc hl,bc, ld a,(hl')) are native, not this set.

Do not use helper call __z80asm__* as the core of a hot hand-written routine. sra hl on this CPU is a helper (cpu_test_vm1_ok.asm); 8085 sra hl is a one-byte op.

Pitfalls

1. pop af discard or move switches MF when bit 3 is set. Only matched push af/pop af.
2. $28 / $38 are prefixes, not LDHI/LDSI. Emitting 8085 ld de,sp+n here is an RS prefix plus whatever follows.
3. 10 is ANX, not sra hl. 18 is sub hl,de, not rl de.
4. OF ≠ K. No jp k after 16-bit subtract.
5. DSUB/DCMP leave OF alone. Do not jp of after them for signed order.
6. call/ret follow MF. Do not change bank across a call.
7. ex de,hl ignores RS. HL′ is not swapped.
8. rla / rra / rlca / rrca do not set Z.
9. 16-bit dec rr does not set Z (8080). No dec bc; jp nz as a count test. Test through A.
10. No copt on library asm. Scan dead moves yourself. tool-copt, methodology-measure.
11. --math32 / --math16 are not vm1 products. Use --math-mbf32.

Preference order (vm1-only code)

1. Never pop af as discard or temp; never park a return address in AF.
2. cp hl,de / sub hl,de / ld hl,(de) / ld (de),hl over byte walks.
3. hl' + RS for a second pointer; stack for a second long (no exx).
4. ld hl,nn / add hl,sp for frames (no LDSI).
5. MB / smf0/smf1 only when the other 64K bank is real.
6. Fall back to 8080-portable sequences for everything else.

Assembler capability (last resort)

File	Meaning
cpu_test_vm1_ok.asm	Assembles; ; comment = encoding
cpu_test_vm1_err.asm	Must fail
*_strict_*	Strict: synthetics forbidden

ok is not always one native opcode. How to rg: skill tool-z80asm.

Related

• Human ISA + errata: doc/kr580vm1.md
• 8080 base (no extras): cpu-8080
• 8085 extras (different map): cpu-8085
• Assembler: tool-z80asm
• Measure: methodology-measure, tool-ticks (-mvm1 before the binary)
• Runtime: libsrc/l/sccz80/7-vm1/, libsrc/l/sccz80/vm1.lst
• Config: lib/config/test.cfg (-clib=vm1), lib/config/z80.cfg